I have a feeling this one might work well if you're looking for cheap...but that's just by looking at Qts. Somebody needs to model it for specific use...

RE Audio REX8D4 (REX-8D4) 8" Dual 4 ohm Car Subwoofer

Over all needs a lot of power and because of it's low output you should run it with a box and give it as much as you can. But still...such small box requirements definitely makes up for lack of free air output