Get access to these tools:
* Drill & bits
* Screws
* Wood glue
* Jig saw
Then look up the cubic feet the manufacturer of your sub calls for (each speaker manufacturer has a different range), and build your own box. Anything you get pre-built won't sound NEARLY as good as a home brew, will cost you 2x as much, and won't really fit right. There's LOADS of information out there for DIY speaker boxes, and they'll step you right through the process. Take your time, think about your build - your efforts will be rewarded.
PS:
Bolt your box down.