How to Make Fiberglass Sub Boxes

  • 4-12 hours
  • Beginner
  • 50-250
What You'll Need
Rubber gloves
Mixing bowl (non-wax coated)
Disposable brushes
Grille cloth
Fiberglass mat
Dust mask
MDF board
Plastic covering
Spray adhesive
Utility knife

Fiberglass sub boxes improve the sound quality of a speaker system. Custom boxes fit right into the trunk of the car, molded to the shape and covered to match the interior of your car. Making a custom fiberglass sub box takes some time, but it is not difficult to create the look of a professionally made and installed box.

Step 1 - Preparation and Safety

Make sure the area you use is free from flames and sparks, including cigarettes. You will need a ventilated area when applying fiberglass. Make sure the area has fans and windows. You may want to remove the carpet in your trunk before beginning work. Use plastic to cover over areas of the trunk you don't want to get resin on.

Step 2 - Cloth and Mix Resin

Put tinfoil on the area you want to fiberglass.Mold it to the proper shape. Tape the tinfoil down. Cut or tear the grille cloth into strips. Put pieces of grille cloth down on the area where you are adding fiberglass.

Smooth out the strips so there are no bumps or wrinkles. Overlap the strips to prevent seams.

Put on rubber gloves and dust mask. Mix the hardener and resin in mixing bowl according to directions.

Step 3 - Apply Resin

Brush the resin mixture on the grille strips. Do not over-saturate the strips. Apply just enough resin mix to wet the cloth strips all the way through. Let dry for 1 hour.

Step 4 - Another Layer of Cloth

Again lay strips down. Lay the grille strips on top of the first layer, overlapping the seams. Apply resin mixture with brush, using a little more mixture to allow for more saturation. Let the layer dry for 6 hours.

Now you can lift the box out of the trunk. It should be hard, strong and flexible enough to remove. After drying repeat Steps 2 to 4 several times.

Step 5 - Apply Fiberglass Mat

Start applying the fiberglass mat. lace mat on the mold and apply resin generously. Do one layer and let dry for 2 hours. Repeat the process and let dry for 6 hours. Repeat these steps until you have a minimum of ¼-inch thickness on the mold.

Step 6 - Trim Mold and Cut Enclosure

The front enclosure will need to be cut out of MDF. The size and shape of the cut will depend on the area of your fiberglass area. You need to cut the MDF to cover the front of the mold as straight as possible. Cut a hole in the MDF to fit your speaker. Trim the edges off the mould so it will fit the front enclosure as straight as possible. Use a jigsaw to make the cuts.

Step 7 -Install Cover

Now you want to place the cover on the mold. Fit them together then use bondo on the mold and enclosure to adhere them together and to build up areas of the mold that may need to be higher. Let the bondo dry. Next, sand the bondo and wood until you have a smooth and seamless finish. Round the edges of the wood.

Step 8 - Carpet the Box

Lay a large, soft, flexible carpet that matches the interior of your trunk face down on the floor. Spray the backing with adhesive Spray the box all over with adhesive, including the inner diameter of the speaker hole. Put the box face down on the carpet. Wrap the carpet around the box, stretching and molding to the shape of the box. With a utility knife, cut off excess in back.

Cut a circle in the carpet that is over the speaker hole smaller than the hole. Wrap the carpet into the inner diameter of the hole. Press and hold until it is secure. Now you are ready to place your speaker and set up your system.