Here's the sub box I built for my SX. It is up against the back seat on the port side, firing into the V drive compartment. I had to go out a couple of inches to get the required volume of 1.25 ft/3 it is 17" wide, 17.5" high. 8" across the top, 12" across the bottom. All dimensions from the outside with 3/4" MDF. Four coats of epoxy and I put bevelled skids on the bottom to keep the bottom off the wet carpet, also out of 3/4" MDF. The amp is in the storage compartment, an 850 watt Planet Audio cheapo US made class D sub amp. Sounds good so far.
I may put a trim panel on it with a grille for the sub, the Polk Momo grille I bought won't work with that speaker. My woodworking skills are not great, but I can make a nice panel to hide my mistakes:)
I may make one for the other side. I would have to relocate the boat speaker amp but that is not a big deal. I could run both subs off the one amp, I will have the gain turned down. If it blows up I'll buy a JL 1000/1 and drive the two W6's
You could make a small box for a good 10 that could probably be flush with the back of the compartment. A 10 w6 would pound pretty good back there, I had a 10W3 in at first that I blew up, sounded great until it popped.
Here is a site that I used to figure the dimensions I needed:
http://www.bcae1.com/spboxnew2.htm