[lunarhighway]

that looks very professional, much better than my effort!

as far as the shape goes there's a number of things you could try, but all in all (and i don't want to discourage you from experimenting) i highly doubt you'll be able to detect a huge difference.

one of the things i'd imagine would happen is that while the air will likely accelerate slightly at the point where the reay slopes down it's got plenty of opportunity to escape sideways or perhaps the opposite would happen, a lower pressure area behind the edge of the tray might actially suck air in from the sides... so perhaps sideskirts could maximise the effect... maybe they won't really boost fe but they should give you some downforce, wich might not be what you're after, but if you can improve a car's handling and fe at the same time that's just an added bonus.

another thing i imagine might happen is that the configuration at the corners looks like it might make vortexes... on the bright side one could hope these work as virtual sideskirts, but interferance with the body likely just turns these into drag... so perhaps some (partial) sideskirts or bending up the angles might improve things.

than again i don't KNOW, most of what i've written here is speculation based on what i've gathered about aerodynamics but i think ,at least in theory, some details might give you an even better result