Fiberglass Box

Fibreglass boxes are often used to enclose radio sub woofers and speakers in the trunk of a car. Although you can have one custom made for your vehicle, it is far cheaper to make one at home. Schedule the time and get the right tools to undertake this project.


  • Staple gun
  • Caulking gun
  • Dremel rotary tool


  • Kyntex 1708 biaxial fibreglass cloth
  • ¾ oz chopped strand fibreglass mat
  • Resin + MEKP catalyst
  • Graduated volume plastic tubs
  • Wooden dowels
  • Tee nuts and machine screws
  • 1 yard of carpet
  • ¼” nylon rope
  • Blue painter’s masking tape
  • 2 yards Lycra (or other stretchy material)
  • 1 roll plastic sheet drop cloth
  • Latex gloves
  • Paint brushes
  • 3M Respirator
  • Cyanoacrylate (super) glue
  • Hot glue gun and glue sticks
  • Duct tape
  • Pam non-stick cooking spray (or release wax)
  • Paint mixing sticks
  • 3M Super 77 spray adhesive
  • MDF Speaker mounting rings

Making the mould

  1. Make a mould of the area you want the box to fit into, using blue painter’s masking tape.
  2. Overlap all seams to reduce the possibility of resin seepage. Use standard duct tape to reinforce any susceptible areas.
  3. Ensure that masking tape is well seated and all areas are covered before continuing.
  4. Mask off surrounding area with plastic painter’s drop cloth.
  5. Pre-cut several pieces of Kyntex to fit the mould.
  6. Mark a line to show where Kyntex will be placed.
  7. Cut Kyntex on a flat surface using a utility knife ( scissors will not work).
  8. Use Pam non-stick spray to cover mould.

Mixing the resin

  1. In a ventilated area, don 3M respirator.
  2. Create a mixture with 4 oz of resin and 2 cc’s of MEKP.
  3. Lay the first cut pieces of Kyntex onto greased masking tape.
  4. Brush on resin liberally and allow Kyntex to soak it up.
  5. Kyntex will not stick to upside-down surfaces. For these areas use chopped strand mat.
  6. Tear (do not cut) off a piece of chopped strand mat that is roughly the size of your hand.
  7. Holding it with gloved hands, apply a tablespoon of resin spreading it to the corners.
  8. Place it directly onto masking tape.
  9. Take a paint brush and stipple more resin onto the mat. This will help it to lay flat and conform more easily to the tape.
  10. Allow it to cure for about 2-3 hours.
  11. Remove the mould carefully.
  12. Build up another layer of Kyntex on the inside of the box.
  13. For flat areas of the box that may be floppier than curved parts, use nylon rope for reinforcement.

Using nylon rope for reinforcement

  1. Hot glue the rope onto the flat areas.
  2. Paint on resin to the rope and allow the rope to soak it up.
  3. Cut pieces of Kyntex to fit over the roped areas.
  4. Hot glue the Kyntex directly to the rope.
  5. Slather on more resin.
  6. Continue adding pieces of Kyntex and, or chopped strand mat until your mould does not flex at all after the resin is fully cured.
  7. Prevent air bubbles as much as possible (bubbles caught between layers of fibreglass will result in flexing).
  8. Trim ragged edges from mould to give it an even edge for mounting against the front side of the box.
  9. Use heavy duty cut-off wheels with a Dremel rotary tool. While doing this, make sure all exposed skin is covered.

Attaching speaker mounting rings

  1. Drill speaker mounting holes into the ring and mount the T-nuts.
  2. Press-fit the T-nuts into the rings with fingers.
  3. Add a dab of super glue under the T-nut’s flange to help keep them in place.
  4. Put the sub into the MDF ring and tighten the screws down to make sure they’re seated properly.

Mounting ring mould

  1. Use hot glue and 3/8” dowels to position the ring so that it is high enough to clear the depth of the speaker.
  2. Apply glue until it is firmly in place.
  3. Let it set overnight.
  4. Staple Lycra onto all sides of the enclosure.
  5. Ensure there are no visible wrinkles on the front.
  6. Apply a layer of resin directly to the fabric (make sure to apply resin where the MDF ring meets the fabric as well as the seam where the front half meets the mould).
  7. Let this cure overnight.
  8. Measure the volume of the enclosure.
  9. Pre-cut enough Kyntex to fit the enclosure.
  10. Apply one layer of Kyntex to flat areas and two layers for curved ones.
  11. Apply resin and leave overnight to cure.

Finishing touches

  1. File down all rough areas and trim off excess fabric using Dremel and grinding tip.
  2. Use a sharp utility knife to cut through resin areas.
  3. Lay out a reasonably sized piece of carpet that matches your trunk’s interior.
  4. Cut a piece of carpet to cover the box.
  5. Starting in the centre of the flat area on the front of the box, apply 3M Super 77 Spray Adhesive. Make sure the carpet is lying flat on the surface without wrinkles.

Tips and warnings

  • Fibreglass, polyester resin and MEKP catalyst are toxic. Use caution and protective measures when working with these items.
  • Fibreglass fabric and chopped strand mat are made from tiny pieces of glass fibres that can get embedded in your skin. Wear long sleeves and gloves while working.
  • The chemical fumes from resin and MEKP catalyst are dangerous to the lungs. Do not wear a paper dust mask in place of a respirator.
  • All tools are available at Wal-mart, Home Depot and similar home improvement stores.
  • The area of the enclosure can be measured with Styrofoam packing peanuts and a 12x6x6 cardboard box. Use the Styrofoam to fill the box and count how many filled boxes are needed to fill the enclosure ie the volume of your enclosure.