Any entry level sub will work to be honest.
If you want something with a small box id suggest the Alpine Type E sub
http://www.sonicelectronix.com/item_...-SWE-10S4.html
250 WRMS and only requires a 0.5 cu ft box...
On top of that its only $65.
If you are going to buy a cheap mdf box...Make sure you coat it with epoxy or rhino liner of some kind. If you build your own, use marine plywood.
Good luck