Its very, very difficult to get subs to sound good with the boot open, they have too much airspace and it unloads the subs, so they flap like hell and generally sound crap, mine will hit the backplate with the boot open.
I'd be looking at buying 1 sub and making a decent enclouser for it, RE audio are excellent, and you might even pick up a second hand XXX for 200 quid, which, nothing is even going to come close too for that kind of money really, aslong as your box and power supply are good enough.