they dont sell boxes like that man... you're either gonna have to learn how to make a good box for 4 15"ers so you can get the best sound, or your gonna have to go to some shop or craigslist around to see if anyone could make one for you. dont expect to find many places or people that will do it. you could always go and buy 2, 15" boxes and just set them in your trunk or connect them together. good luck man.
