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Fiberglass Box

Building a fiberglass box is a challenging project that may take days to finish. But since it may be the only way to customize your subwoofer box, it is worth a try. You can make a unique subwoofer or speaker box that suits your taste and needs.

Materials needed

  • Fiberglass mat
  • Resin and hardener
  • Auto-body filler (bondo) and hardener
  • Plastic sheeting, aluminum foil, or other molding material
  • Pencil and paper

  • Tools needed

    • Paint bush
    • Sander
    • Multi-purpose shears
    • Screws
    • Wood saw
    • Angle grinder


    Fumes and dust particles are generated when you work with fiberglass, so you have to use a respirator or dust mask designed specifically for it. You should also wear gloves and protective clothing when handling fiberglass and resin, and also when sanding work in an open area.

    Steps to Make a Fiberglass Subwoofer or Speaker Box

    1. Preparation


      2. Begin by planning what you will do. Know the requirements for space and the volume of the speakers and components. Measure the space in the dashboard of your car if you are inserting your box there. It is best to do a layout in three dimensions.

    2. Molding


      3. If you need to make a particular shape, you must make a mold or pattern. There are different ways of doing this. You can make a frame out of wood, aluminum foil, or chicken wire. You can also use modeling clay or shape dried spray expanding foam.

      When your pattern is ready, apply mold release and aluminum foil to prevent the fiberglass from sticking.

    3. Laying the Fiberglass


      4. When you are ready to lay the fiberglass, combine the resin and the hardener. It may take a little practice to find the right ratio of resin to hardener. Too much hardener will dry it at once, too little hardener will take hours to dry. After doing this, cut the fiberglass mat into the proper shape.

      To apply the fiberglass, you can either dip it in the resin/hardener mixture, or coat it with resin by brushing. Place the first layer of fiberglass over the mold. Thin strips work best for corners or small projects, and sheets work best for longer and wider projects.

      When the first layer is dry, cut and sand the excess fiberglass. You can now add the second layer. Make sure that there are no air bubbles, wrinkles, or pits. If this is not done properly, it may warp later.

    4. Putting Bondo


      5. When the shape is defined and sturdy, smoothen it by sanding the rough edges. Working with bondo is similar to working with fiberglass. The key to applying bondo is using thin coats and more coats than you think you need. Spread the bondo over the panel, dry it, and then sand. Do this until all the gaps are filled.

    5. Finishing


      6. The last step is the finishing. If you plan to paint your project, you need to apply glaze coat to fill in pin holes and dimples left. If you will cover it with carpet, you don’t need to accomplish this step.