Honestly, if you want to build a subwoofer, go get a job at Sony. A good sounding speaker cannot be built at home. The best a do it youself guy will be able to make is 2 pieces of tin that vibrate and make a humming sound.
If you want a box, I built one that fits in the 2nd gen trunk, and the back hatch has to be opened to take it out. It rocks. It's so easy to break into the 2nd gen doors (I did it the other night in less than 3 minutes with a piece of metal that i found on the porch), but opening the hatch without a key is quite a job.
I have designs for the box, THAT SOUNDS 10X BETTER THAN THE CHEAP BOXES AT BEST BUY. It has nearly 30 inches of air travel before it hits the outside world, and is able to accomodate any speaker size or specs by just lengthening or shortening the sides/top/bottom. It hits harder, and sounds crisper than any box I've EVER heard.
I also have designs for a scort designed box that incorporates mirrors, neon, and has 36 INCHES(!) of air travel before it gets outside the box. THIS BOX WILL WIN MEDALS! I will sell the plans for around $10. The box will cost you an avg $80-100 to build.