[Sven7]

Sounds like a good plan. I wonder how much it will wiggle side to side due to the loose tolerances in hitch design. Maybe you'll have to do some sort of straps around the truck's rear hatch to secure the boat tail. Go for it, and report back!
Make sure you have a belly pan first though, as it will help the boat tail function properly. You're trying to move the point of flow separation back a few feet, and if the air is already separated it does no good.

With a box shaped rear end you'll want a zero degree angle on the rear of the belly pan. Completely horizontal- any upward slope will increase drag comparatively. I'm not sure how far back this applies so you may want to ask Aerohead and study Basjoos' AeroCivic.