I have a BOSS Audio 12" subwoofer, rated at 500 watts RMS. This is a nice subwoofer if you don't have a lot of space. The subwoofer is thin so can be mounted in a thin box. You could probably go under a cubic foot if you use poly fill inside the box. It is a 4 ohm subwoofer, with a frequency response of 28Hz-2KHz.

This would make a great addition to your car stereo system, and obviously great if you are starved for space. I will sacrifice it for $35. BOSS makes good stuff.