If you list the available space you are willing to give up H x W x D and let the forum weigh in on choices, you'll get good info. People here will tell ya what'll fit, even account for port clearance, I'm sure everyone here would be able to advise you on what'll fit, get low and sound good... just let folks hear what sort of result/goal you want. Also, if you put up a pic of the cargo area that would help. If you want to keep it under the windowline make sure the height you give is that measurement, and realistically how many electrical upgrades are you willing to pay for. A 3-5K amp will probably need some $$$ just in electrical upgrades to be done correctly (alt, batts and wire) so think about that.
Even a single 15 can use just a bit of space...
Lol... doin a pair could get kinda big... do you got a lot of space that you'll give up?