i've always wondered about modern sedans. the rear glass transition always seems smooth enough. that I bet the air stays attached, even though its sub template and producing a low pressure zone (acting like an airfoil)
And as long as you have a horizontal or convex surface at the end to turn the air flow again and produce a high pressure zone, it might work better than a kamm back.
case in point is the new mercedes CLA which gets an absurdly low Cd and even the prius which is sub-template
So for your car, maybe all you have to do is extend the horizontal of the trunk out closer to template
