Build it. Get the specs of what you want the sub to do, and decide on the size, and type of box you want to use. Then build it, its not hard, maybe take a couple of hours, and you'll feel a lot more accomplished when its done.
Depending on if you want more along the lines of tight bass, or deep bass, you could go with sealed, or ported respectively. Now before i get flamed, i know that a ported box can still sound tight and all, but generally its going to be tighter sealed. Plus, sealed is easier to build. A larger box will be more boomy in the sound, and a smaller box will give you a tighter sound. So depending on your listening preferences you can make your decision.