There is quite a lot of room in there behind the bumper. Your could get a 2nd hand Mitsubishi VR4 intercooler for around $350 - $400. I am using a R33 Skyline GTS-T intercooler (because it was given to me) which is approx 30cm x 30cm and which seems to cope with 15psi OK. Although no doubt it is a little restrictive and/or not cooling the air as much as ideally I would like. They would be cheaper-ish, like $250 or less I imagine. Another option is Supra intercooler (7MGTE motor I think)...