Something like this... but will need to check how contours of the truck goes and do some estimations of how much "curvature" could be....
Blending all surfaces nicely to the back of the bed (not increasing wake area) and keep transitions of surfaces following closely to the truck's overall shape is the target.... Once that is established, then tweaking surfaces here and there is much easier... taking baby steps while learning....
https://ecomodder.com/forum/member-t...-211524-02.jpg