If you want advice on enclosures check out
Steve Meade Designs (SMD)
On average a 15 requires 3 cubic ft of air space. So getting 12 cu. ft. in a cruiser is going to be tricky. If you are on steve meades site or ptcrew hit up a guy that goes by ptcary he has 8 12s in his cruiser. He should be able to give you some advice.