The thing about the Morelli ring vortex is it is 360° with special attention to the wake coming off the back tires. It actually has turbines in the wheels that pressurize slots at the ring. The pickup bed tailgate hinge line falls about the middle of the ring.
[a]erohead has built pneumatically inflated tailgate add-ons. Check his albums.
http://www.boatdesign.net/forums/materials/pet-g-mylar-film-46223.html
I would do 2 or 3 layers of fiberglass, hit it with 80-grit sandpaper, and then do the last coat or two using Mylar. I always though you puddle it and squeegee it out, but the linked article suggests a toothed applicator.
There is some discussion of trapped air bubbles. I'd try pricking the Mylar to let the air out in that case.