Do kind of like I did with my
SedanKamm, start out mimicing the car, and then go to a "chamfered" back end, which mimics round. My top angle started out at about 10deg and went a bit steeper (maybe 15deg?) about half way down (it had a post stiffening it in the middle). The sides can come in at the same angles IF you have good attached flow on the sides like you do on the roof. Otherwise take it a bit more easy. Test for attached flow with a tuft test. They are easy to do if you have windows, which I would suggest so you can see when driving. Build in and around all tail lights, as Phil said.