What sub are you looking to go with, this will decide whether you can simply mount it to a wall, or need to build a box. The pics in the link really didnt show much, I would look at the driver's footwell, pass footwell, or walk-thru wall. Next would be to build a box for a storage area. As far as the wall goes, look for a wall that's large enough for the sub, thick enough to support the weight of the sub, the area is deep enough to mount it, and you have access behind it to run wires.
I would recommend the WetSounds XS-10FA and the Syn4 to power it and your 4 in-boat speakers.