The Cd and aerodynamics on this car are quite good to begin with. I don't think anything you do will help your car short of a 2 or 3 foot extension which would have to be very well designed. If you do anything, just be sure that the transition is gradual and it must be a radius.
Stay on these green lines. This is a full boat tail idea, you'd want to make it shorter for practical reasons, but anything you do shorter than a 2 or 3 foot extension would probably not be worth the effort.
