I think about it all the time and one day will get off my lazy a$$ and build one for my car. In all my thinking about it though, I have determined you need to do it to the template or not at all. Chances are whoever designed your car has balanced out the air going over the back of your car to minimize any kind of vortex creation, if you try to do something half baked, you'll ruin that and end up with a worse Cd.
Here's a photoshop version of what I wanna do.
What kinda car ya got??
