If it were mine, I'd be trying to emulate the following shape (excuse the rapidly made craptacular 'shop):
I hacked on the back half of a prius, rescaled to the same length as your roof peak to tail end. I grabbed the point where the rooflines both become level. I also put on a bloody great air dam, because its the ecomodder version of putting on a flame job in photoshop
It might be acceptable to simply slope from the rear of the roof to the tailgate top, though. It appears that's how Phil Knox did it, and I don't pretend to know nearly as much about aero:
But I wonder if there is a problem with maintaining attachment over the transition from roof to slant back. IF there is, some VGs might help just forward of the transition, or perhaps the more gentle slope of the curved prius-style kamback would be better.
Anyone with more aero knowledge care to do a mock up? Here is an excellent Tundra side shot:
