Subwoofer boxes come in a variety of styles, including sealed, ported and bandpass. The sealed enclosure is probably the most widely used and the easiest to construct. It is commonly a rectangular or square box with all of its seams sealed with silicone. (The box must be sealed so no air leaks out of it.)
Consult with the manufacturer or retailer of the subwoofer speakers you have purchased to determine the proper dimensions of the enclosure. Make sure you account for the thickness of the particle board (3/4")
Use a pencil and tape measure to trace out the patterns of the enclosure on the particle board. You will be tracing out the patterns for the six sides of the box, plus a center divider for inside the box to separate the two speakers.
Using the jigsaw, cut out the patterns.
Using the grille cover for the subwoofer speaker and a pencil, trace the outer and inner portions onto the location of the board where you intend to place the speakers.
Take the router with a 3/4-inch bit and set it for a cutting depth of 3/8-inch. Route out the area between the inner and outer circle that you just drew. This is where the frame of the speaker will rest flush on the finished box. If you don't have a router, drill with a large drill bit to make a hole. This will serve as the starting location for your jigsaw. Jigsaw and cut out the hole of the inner circle. In this case, the frame of the speaker will rest on top of the board.
Drill a hole with a large drill bit. This will serve as the starting location for your jigsaw. Using the jigsaw, cut the holes for the speakers.
Use a file to smooth the edges of the hole and sandpaper to smooth out the router area.
Cut a 2-inch round hole in each of the side panels. Connectors for the speaker wires will fill these holes on the finished box.
Assemble the box using screws, and seal the joints with silicone to ensure that air will not leak out. Leave the front panel - the side that will hold the speakers - off the box.
Spray glue on to the outer portion of the box and the back of the carpet, and wrap the carpet tightly onto the box.
Apply carpet to the top panel (the one that will house the speakers).
Using the razor blade or scalpel, cut vertical, horizontal and angled lines in a star pattern into the carpet that covers the holes for the speakers. Cut out the carpet that covers the connector holes.
Place the connectors through the holes, so that the two prong terminals are inside the box.
Screw down the connectors and seal around them with silicone.
Solder the speaker wire to each speaker and connector. Solder the positive wire to the positive lead of the speaker, and the negative wire to the negative lead of the speaker.
Put the front panel on the box with screws and seal with silicone.
Push each speaker into the star pattern cut out on the carpet that covers the speaker holes on the front panel.
Install the box in your car.
You will find most of the tools and materials you need at a neighborhood hardware store. The wood can be purchased at a lumberyard or local home center. The store where you purchased the speakers may have computer software that helps it design subwoofer enclosures. Ask the salesperson to print out a sealed enclosure designed specifically for your speakers and car. Silicon and silicon vapors can damage your subwoofer. Make sure the silicon is dry before putting the subwoofer into the box.