Oh cool !
Glad that could be of use.
You are just mocking up pieces now aren't you ?
The reason that I ask, is that I see that even in your sketches, you have a lot of flat sides and sharp angles.
I'm assuming that you are going to smooth those edges right ?
I'm sure that you know this, but the angles that you
have now will result in two
huge vortexes ( corkscrews ) of air at the rear of your car.
The rear of the tail should have rounded edges ( similar to my first illustration ) versus being a boxy shape. The red lines in the picture above show the path that air will take in your current configuration. The black lines are what we are after.
It is basically just a tapering of the shape that proceeds it.