I have been researching the F40 style rearhatch for some time now, but I dont know how to make these shapes in plexiglass only...plus I am looking for a cleaner style for the "stock" fastback and trying to make it relatively easy to build.

The idea is a twin-hatch: stock decklid with the middle piece cutout and an added fiberglass piece (between the sailpanels). The seperate top piece has a fiberglass frame with a piece of "bendable" plexiglass plate that can be screwed or popped in place. It is hinged to the rear roof. Ideally it would be nice to have some linkage between the upper/lower hatch so you could open the decklid the normal way and the top hatch would move up a bit more so they dont hit. A one-piece design would be much more difficult to build because it needs to be hinged at the roofsection which is to weak to bear all that weight without reinforcing.
Ow btw, I will also try to incorporate a row of small vents on both sides (like the newer ferrari's) so the rest can stay smooth and clean, if you know what I mean

Are your gears still turning?

this would mean a fiberglass or metal section at the top with a few louvers the full width (about 4 inches worth), a narrow single band of "vertical" openings about 1" wide along the sides (to avoid blocking vision), and a single row along the bottom edge.. this equated to more open space in areas with better pressure differential and would also help prevent the slope of the window from generating lift.. I can scan the drawing if you are interested.