I say build it yourself. It really isn't that hard. I am terrible with tools, but I have built two solid boxes. Been running one for about two years now. If you do not have a table saw, go to a lumber yard. They do pretty good cuts. When I built my first box after buying the wood and the tools specifically for building the box it cost me less than a prefab. My last box cost less than $20 to build using 3/4" MDF.
Sounddomain's subwoofer section has a thread showing how to calculate tuning as well as displacement for port, sub, etc.