Sub-woofer boxes can be bought from many retailers, but most vendors don't offer boxes that hold four subs. Building your own custom box can save money and allow you to pick your own size, location and look. Using a few simple tools, nearly anyone can build a custom box that will hold four sub-woofers.
Find the recommended enclosure volume from the sub-woofer's manufacturer. This information will be located in the sub-woofer's manual or can be found by searching online.
Measure the height, width and depth of the area in the vehicle where the sub-woofer box will be located. Factor in any obstructions that will complicate fitting the box inside the vehicle.
Calculate the volume of the sub-woofer box using the measurements from Step 2. Subtract 1.5 inches from each number that was measured. This accounts for the thickness of the 3/4-inch medium density fiberboard that will be used. Multiply the height, width and depth of box, and then divide that number by 1,728. This will provide the internal cubic foot volume of the sub-woofer box.
Compare the measured volume from Step 3 with the recommended volume. If the measured volume is larger than the recommended volume, adjust the measurements down in equal amounts from all sides of the box so that the volume decreases.
Create a listing of how many boards you will need of each size and use a table saw to cut them off the sheet of 3/4-inch medium density fiberboard.
Cut four circles with a router or a jigsaw in the board to which the sub-woofers will be mounted. Cutting templates and/or hole sizes can sometimes be found in the sub-woofers manual or on the manufacturers website.
Glue the medium density fiberboards together to form the box that is based on the measurements made in Step 4. For extra support, use nails or wood staples to seal the boards together. Let the glue set for a few hours to ensure proper adhesion.
Cut a smaller hole for a terminal cup on the opposite side of the box from the where the speakers will be mounted. Terminal cups can be found online or at car stereo shops.
Apply carpet with carpet glue or staples. Cut holes in the carpet for the sub-woofers and terminal cup. Box carpet can be purchased at most retailers in the automotive section or at car stereo shops. Carpet is not necessary but many people prefer using it to create a more factory look for the sub-woofer box.
Insert the terminal cup into the small hole and secure it with screws. Connect a piece of speaker wire to the positive and negative sides of the terminal cup to allow the wiring of the sub-woofers.
