you could always make it yourself for about $25 in supplies. Especially if you want a sealed box its very easy to make them. The D9 prefers a ported box according to the specs on Diamonds website. Its a bit more complicated to build, and alot more complicated to design.

If anyone here is more familiar with building boxes for them they may be able to tell you the difference in performance between the subs in ported and sealed. The installs I've heard with the D9 were all ported.