I made my own enclosure, and it was well worth it. I borrowed most of the tools from friends/relatives, and bought the MDF from Home Depot. A 4'x8' section was something like $25 (can't really remember now).
I calculated the dimensions that would best accomodate the sub and a couple of internal braces to make a 2ft^3 sealed enclosure, which is what the manufacturer called for. I took the MDF to a local cabinet maker who cut the sides and braces. I had them do it because I wasn't confident that I could be accurate enough with a rickety table-saw.
I cut a hole in the braces that made them roughly 1 inch thick all the way around. I used carpenter's wood glue and 2" drywall screws to mount the braces to the inside and connect all the sides together. After letting the glue cure for a day I used silicone sealant on all of the joints from the inside of the box, and where the wire goes through the back.
When the box was done I (200+ lbs) could easily jump up and down hard on the box, and it didn't even budge, not to mention I could probably fill it with water and it would hold. No, I'm not willing to try that out!
