How To Make Fiberglass Subwoofer Boxes? Top Full Guide 2022

How To Make Fiberglass Subwoofer Boxes? Top Full Guide 2022

How To Make Fiberglass Subwoofer Boxes? Top Full Guide 2022

Most car enthusiasts know that having a good set of speakers is essential for adding life to music. However, many people don’t know the right way to prepare their vehicle’s interior to do so. One way to do this is by making custom fiberglass enclosure boxes. These boxes can be placed anywhere in the vehicle and they won’t be hard to put together either. 

So, How to make fiberglass subwoofer boxes for your car? Keep reading to get the step-by-step DIY guide.

Material Needed

  • Polyester Fiberglass Resin – 1 Gallon
  • Fiberglass Matte
  • Additional Resin Hardener
  • MDF (Medium Density Fiberboard)
  • Dremel / Rotozip / Cutter
  • A GOOD Respirator
  • Disposable Paintbrushes Brushes
  • Masking Tape
  • Tin Foil
  • Glue Gun or Adhesive Spray
  • Fleece Fabric
  • Wooden Dowels
  • Carpeting Materials
  • Packing Peanuts
  • Wiring Materials
  • Light Sandpaper

Additional Information

This project is not easy. It will take about 30 hours to complete, dry, and adjust. To get your car ready for the season, you should expect to spend at least 8 hours working and have a good view of your trunk space. It would help if you ended up with a stunning sound system that looks as good as it sounds.

How To Make Fiberglass Subwoofer Boxes

Step 1: Remove Everything From Trunk

Remove Everything From Trunk

Now you will need to get rid of everything in the trunk. Even the trunk lid should be taken out if it is getting in your way. You can leave the carpet intact, but you’ll need to make sure it conforms to your car’s contours as much as possible. You don’t want your trunk to get clogged up with dirt and debris.

Step 2: Tape Layer

Tape Layer

Use high-quality masking tape (the blue painter’s tape is the best), and begin to apply tape to the area you are going glassing. To prevent resin seepage, I recommend you make two layers and cross them. You can’t get resin off metal, carpet, clothing, or your dog. You should extend more layers about 3-4 inches beyond the box. Glasses can also be taken out a few inches beyond the box but within the taped area.

Step 3: Tin Foil Layer

This is something I’ve seen in tutorials, but it is up to you. It adds an extra layer of protection against the polyester resin seeping through to your carpet. Tape and overlap the aluminum foil squares as you go. Now you can start glassing.

It is vital to protect the interior as it will not be damaged. You can place newspaper or plastic around your work area. To prevent any fumes from escaping and smelling up your car for several days, you can set a sheet of plastic between the trunk and cabin.

Step 4: 1st Resin And Fiberglass Layer

1st Resin And Fiberglass Layer

This is where the fun and most of the time goes into this project. It will take forever, so you should take your time.

NOTE: You must wear a mask for your face, but not the disposable type. The resin and fiberglass fumes can get into your lungs and cause breathing problems. You can get a high-quality one at Home Depot and use it in an area that is well ventilated.

Mix the polyester resin according to the instructions. You won’t want to use a large amount or alter the hardener amount (you can adjust it depending on your temperature). It is best to use between 2 and 4 ounces at a given time. Mixing a batch in plastic cups is a good idea. Then, work in small areas. It should take you about 30 minutes to complete the task. You will need to cover a large room with resin and a hardener.

After you have prepared the resin, prepare the fiberglass. It’s not complicated. Just rip the matte into small, manageable strips about 1 “x4”. Wear your mask and disposable gloves!

You are now ready to start. If you’ve ever used paper mache, the procedure I used was very similar to it. Dip your brush in resin and saturate them, but do not soak them so much that you drip resin everywhere. Next, please take one of the glass strips you have prepared and place it there.

Use your brush to apply resin again to the strip until it becomes transparent. It would help if you were looking out for bubbles. If you see white spots bubbling up, you may have air trapped. To get rid of the air, use your brush to spread it out. If your subwoofer box is too weak, air bubbles can cause it to explode.

Continue to lay fiberglass in this manner, adding more pieces as you go until the layer is complete. Remember to make your shell larger than you need. You will be able to trim it later. It should dry for about two hours before it becomes touchable. You can speed up the drying process by using a hairdryer!

Read more: Best Subwoofer Box For Car Trunk: Top Brand Reviews 2022

Step 5: Additional Glass/Removing

Additional Glass/Removing

You’ll need to add more fiberglass layers using the same method as the previous steps. It may be possible to remove the fiberglass from the vehicle after a few layers. However, it will be much easier to work outside the car. It is possible to have trouble keeping the trunk’s contours in the box if the glass isn’t strong enough.

It is time to remove the cardboard box from your car. It’s okay if some of the tapes stick to your vehicle; it won’t bother you. Now, you should be glazing outside the vehicle. Make sure that everything is dry before you put it back in your car.

Step 6: Trimming the Excess

Once your fiberglass layers have been laid, you shouldn’t have the sharpest edge on the outside of the box. You probably added more fiberglass than you intended. If you want to achieve a suitable thickness, you will have to trim the excess. You will need to place your box in the trunk. Make sure it is snug. Draw a line around where you want the edge. Next, use a Dremel or roto zip to trace the line. The result should be a neat edge, evenly sized throughout.

Step 7: Working Around Obstructions

My vehicle has a moving part that could contact the box when the trunk is closed. You should make sure to note any obstructions in your chest and allow for clearance. This is an ideal time to do this before you start installing your speaker rings. To create a pocket inside your box, make a jig from MDF.

Use hot glue and wooden dowels to fix the piece where it meets the obstruction. After it is secured, you can either fleece or fiberglass the area surrounding it (step 10) or wait until your speaker rings have been installed. You can do whatever you like, but I suggest that you start by clearing out the area where there is obstruction. It will make it easier to lay fiberglass inside.

Step 8: Speaker Rings

Speaker Ring

This process is not something you will enjoy. Your speaker rings will be made of MDF, preferably 3/4″. You will need to have a diagram of the dimensions and cut according to your subs. You can add a second ring to your subwoofer enclosure box for a different look. It will look great if you attach the two circles. You should make sure your ring is more prominent than usual if you plan to use carpet or countersinking. If your circles are visible, you may paint them black. You can find additional information on speaker rings elsewhere on the internet.

Read also: How To Build A Subwoofer Box? Top Full Guide 2022

Step 9: Positioning And Volume

Now you need to consider how much volume your speaker box will hold. This could cause problems if you don’t have enough space to fit the sub you want. Packing peanuts are a cheap and easy way to find the volume. You will need to take your subs’ preferred volume and wheel wells fill your box with the same amount of packing peanuts.

Place your speaker ring where the packing peanuts are. You will need to place your subs coil a bit farther out to make room for your speaker ring. It is better to have too much space than not enough. Move your speaker ring to that location and think about where it was. Now you can decide whether to position it towards the front, trunk, or upwards.

Now comes the tricky part. You will need your hot glue gun and dowel rods to attach them to opposite sides of the speaker ring and to the fiberglass enclosure. You can also use plumbers metal positioning wire to support it, but that is just as labor-intensive. You don’t want the ring to fall or come loose. Attach them to opposite sides of the speaker ring and to the fiberglass enclosure with a hot glue gun.

Step 10: Fleece


Fleece or another cotton/polyester fabric will be needed to cover the large opening between the fiberglassed pieces and your speaker rings, MDF, or other components. Attach the fleece to the top of the fiberglassed area. The fabric should be stretched over the MDF and secured with hot melt.

Now you should have a material that covers everything not covered by fiberglass. You should remove every wrinkle from the fabric. It is important.

Step 11: Resin


Use the excellent mixture, the resin, and the hardener (as mentioned in step 4), and get your cheap paintbrushes to start painting the fleece with resin. You can cover the entire area except for where your sub will be, but that’s unnecessary. The resin should be poured on the fleece until it is completely saturated. It will take some time to dry, but it should become solid once it has dried.

Step 12: Fiberglass

Now you are done with your fiberglassing. Follow the steps in step 4. Add 4-5 layers of fiberglass to the fleeced area. You should not add glass to the place where the subs are. These will be removed later.

Step 13: Trimming the Fleece

Now you can cut the sub hole and trim your fleece. To open the area, use a Dremel tool or another similar to shorten the fleece as close as possible to the MDF speaker rings. If you have any material left behind, you can remove it.

Step 14: More Fiberglass Support

You may notice any areas on your box that are weak, such as air pockets or thinness. This is a good time for fiberglass to be added. To keep the outside looking nice, you should do it on the inside.

Step 15: Sub Wiring

There are many ways to wire subs. Problem number one is how to power the subs after they are sealed in their boxes. Pre-made boxes usually have a connector that you can connect your wires. It won’t take long to put one in if you don’t have such a box. You can also wire a small pod into the instructions in step 7.

A simple pod is another option. Simply drill a hole, insert the wires and seal them. You can also use connectors that go through fiberglass and connect on the opposite side. It is up to you. Just make sure the holes are air-tight.

Step 16: Sanding

You don’t need to sand if you did this correctly, making sure that all fiberglass was smooth and free of bubbles. Also, make sure the fleece is wrinkle-free. There may be some humps or areas that need to be sanded. You can lightly sand it, but make sure to wear your mask! It would be best if you continued to sand the area until it was completely cleared.

Bondo or more fiberglass can be used if you have to file in a location. It is not necessary to smoothen the carpeting, but it is essential for vinyl and painting.

Step 17: Complete the Box

Fiberglass Sub Box

You can customize the appearance of your box in many ways. You can paint the outside of the box any color you like, apply vinyl to it, or carpet it to match your home’s interior. This tutorial will focus on carpeting since that is what I chose to do.

You should find automotive carpeting easily. Some hardware stores sell carpet that matches your interior. Just go to your local home center and take a look. However, I needed the same carpet as in my car, so I went to the junkyard and took the carpet from a scrap car. You will only need to clean it up. It is easy to attach the carpet to your box. Once you have cut it to fit and checked that it fits, you can use a 3M adhesive spray to stick it to the box.

Step 18: Volume Check

Use the packing peanut method again to check the volume. Your box should be larger than the suggested size. If your box is too small, I don’t know what else to say except that you can live with it or make a new one. If you have larger Subwoofer enclosures boxes than the recommended size (which is probably most of you), you can add Polyfill to fill in any cracks or odd sides to reduce volume.

Step 19: Crank Up The Beats.

Fiberglass Speakers Box

Now you can install the sub in the hole and wire it up. Finally, install the box in your car. It should fit perfectly, be snug and secure. I don’t know how to help you if it’s not. To secure the vehicle, you can place screws in the box and the frame to ensure it is close. It might be worth looking into a sound deadener that you can place inside the trunk or behind the box. This will stop any loose items from bouncing around in your box. Turn on the amp and turn up the beats!

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