Build my own? How would I do that?
Car audio subwoofers are cheaper than an equal home audio one, but they are the same thing. So id start there.
http://www.sonicelectronix.com/item_33741_Rockford-Fosgate-R2D2-10.html
Something like that would give you alot of boom in the right box. Building a subwoofer box is rather easy and cheap if you got the right tools:
1. Table saw
2. Circular saw to cut the hole for the subwoofer
3. Wood glue and thick wood screws
4. Medium density fiberboard you can buy at home depot for really cheap.
And an amplifier will be another matter. But if you dont wanna take the adventure of building a box you can always buy one made for car audio and rip the carpet off and sand it down and apply vinyl to it and it will look just like a normal home subwoofer box.