Has anyone...

[Crash ?]

Building a box that fills in the back of the truck to put the amp in would work.. Mount the sub box on top of the platform with quick release connectors and wiring disconnects so you can get some of the space back if you need it.. Heck.. If you can figure out a good way to mount the subs in that deck box, fire 'em straight up and still have them protected physically and visually you wouldn't loose much space anytime..

Rob