bondo was working on a semi box cavity that attached to the rear of the cab. Not sure where it is in the process of production. Maybe you could give him a PM.
http://ecomodder.com/forum/showthrea...ote-35533.html
Lund made a racer cab spoiler that you could borrow from the design. It was discontinued and very hard to find.
https://www.pickupspecialties.com/lu...er_by_lund.htm
You can see something similar with the Toyota T150 concept (became the Tundra) which was bed-based instead of hanging off of the cab.
Maybe these examples will give you some ideas in which direction you can go.