Like this?
A simple curve can be defined by 3 points. Any arbitarary curve can be defined by four points (B-spline curve). Your problem is going to the third dimension. Here's what I did: Use any limber, uniform material PVC pipe, bamboo, etc. I used redwood bender-board, 'cause I had lots of it.
Make a scaled down [in x and y] outline of the cabin rear profile and place it on the rear deck and bend your material over it, and then slide it back and forth.* It's like the street rodders say, "If it looks right, it is right."
For what gain you will get, like aerohead says, you might want to focus on a front diverter and front wheel well air management. Or rear skirts.
Edit: *Start at least a foot ahead of the point of separation to get an accurate tangent. I started at the stagnation point in the front to explore what happens across the roof after the rope at the highest/widest point.