On my 05 I had a box made since I didn't order the sub with the boat. On the 07 it is in the same spot as the pic and was ordered with the boat. They simply do a cut out at the factory on the forward part of the floor and screw it down. Just like the rest of the speakers are installed. That is where most subs are put and it sounds great. I'll take a pic of mine and post today. If you have your Moomba manual(or brochure, in back)it shows the sub pic in it also.
Here is the custom box I built with 2 JL Audio 10W-3's. The only thing I noticed after I finished the system was that the bass was making everything in the instrument panel rattle. I purchased a sheet of dyno-mat and stuck it to the underside of the fiberglass console and it cured my vibration issues!