A short-box reg. cab chevy silv. 1500's bed is a square 50.6w * 68d * 21 vert. height, with a tonneau cover it probably goes up to 30"+ in height, grossing nearly 60 cubes, around 52 with 1" MDF. Would that not be like the ultimate hatch because you could use a 4th order and have the port the same size as the rear window (obv. not attached but pointing at it)? You could use 8 18's in around 6 cubes each (3 cf sealed/3 cf ported). Spray inside the entire bed & tonneau with deadener . A long box would grosse 86 cubes.

 

its been done before on the SMD forum. people have done a blowthru via the rear window rather than cutting the sheet metal to the bed of the truck

 

Impossible to find with google.

 

Try to find on SMD forum