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The underside of the Flea's boat tail angled up to 6.5 degrees, and retained attached, if turbulent flow to the trailing edge, documented in the tuft testing video.
I think basjoos is right: you can get away with steeper angles if you have adequate radii between sides/top and sides/bottom plus taper in both plan and profile. That would permit pressure recovery from the sides, and the bottom angle can be increased.
My taper used predominantly straight lines on the top & sides, but if you arc gently through the taper, you can go up to 22 degrees and retain attached flow, says Mair (says Phil). The Insight has decent enough upstream aero that I bet it's possible on your cars.
The challenge in construction for you guys will be the unusual cross-sectional shape at the rear of the car. You've got a lot more taper already in progress in the greenhouse at the transom than you do at the height of the rear wheels.