Both my vehicles have problems in this area. The Beetle is pretty hopeless, that's the area the engine cooling air comes from.
Isn't the rear roof angle about the worst it could be? I'm thinking that an extension should be fairly short across the top, but with sail panels down to the beltline. Creating vortexes that roll onto and reattach to the window glass.
At least I don't have taillights to deal with like the Roomster and Insight.