Okay, I've got some basic plans. I'm going to Prius my rear end.
I'll have the angled Kamm extend out to the end of the trunklid, and a flat spoiler extending to meet the side extensions, with a couple pieces of coroplast to have the sides meet the spoiler, and have the rear edge all on one plane. Then I'll take a sheet of clear acrylic and cover the entire rear end so it doesn't end up acting like a suction cup, but a clean, flat surface.