When I originally suggested this thread (which was split off from another) my intent was to see more information on diesel_john's car. However this thread got titled with my own car's description. Both cars have a large rear face with the large wake that generally produces.
Anyway, based on the guideline of maximum 15 deg. angle to maintain attached flow, I did two drawings. I didn't measure the drawn angles but I think it's close enough for where I/we are now. You can see that to reduce the rear face appreciably on this car you need a reaaaaaaallly long boat tail.
I don't know if I dare build this thing. It would have to be pretty large, which probably means heavy, and also will have a large side profile to catch side winds, with all the construction issues that come from those issues.
Rear view:
