I can't begin to tell you how much time I've spent futzing around with different ideas, trying to find something that works and doesn't mess up the overall lines of the car. This is a VERY well designed shape, and it doesn't take much to screw up the looks.
Here are the only things I've found that I think might work, if you want to follow through with the side stripe idea. I haven't been able to find anything for the back of the car yet that I like though.
This one I pulled the stripe from under the license plate, so the stripe stops at the sides.