What size box should I have to maximize SPL?

Okay so I am having a custom box made for my 15" Memphis Mojo (15-CM15D2) I'm going to be pushing it with a 2k Memphis Mojo amp. I currently drive a 2000 Grand Prix GTP, so its a trunk bumper. I want to find out the shape, cubic ft, and design that would be the best for my vehical and other sedans. I would like to be competition capable if I ever find some money worth dropping into the system.