Ideally, the trailing edge should taper down from full thickness at a 15° angle. Needless to say, there's no room for that.

This is also a favored shape for the back of LSR vehicles. The "Kamm" principle dictates that if this isn't possible, the next best shape is to reduce both length & width of the tail at 15° until the cross-sectional area is 50% of the car's full dimensions, and cut it off flush at this point (no radius). Is the closed end vertical, or 90° to the air flow? IDK.