I am mulling over getting an 18" SSA XCON, but I'm not sure I realistically have enough space to make it sound good. I have a '98 Jeep Cherokee. I can fit about a 40"w x 22" h x 23" d box. The depth of 23" is where it runs right up against the hatchback, so I assume the box would have to be reasonably less than that. Now, at the base of the box, the depth could be around 28".....the back seat tilts back, so there is less room at the top...23" top, 28" bottom.
As far as sealed vs ported, I have no preference. I have heard that I might want a ported box on an 18" sub, but I just want the best SQ. I'm sure it will get loud enough for me.
Here are the recommended enclosures from SSA for the 18" XCON...my available dimensions are a little different.