Space is not a problem. Really...I could rip out everything if I wanted to and just make a flat part in the box to sit onHow about a single SX18 and an opti4000D
with a 5 cube box with 150 squares of port, and a good electrical, you should be nipping 50's fairly easy
straightforward setups to hit 50's are gonna cost a lot, you could do it for a lot less, but you gotta substitute experience, knowledge, and trial & error for $
you could also get louder with a bigger box, but i was too lazy to see if you already said if the back seat can go or not...