I'm curious at how a duck tail wing will look on a coupe. I think it is possible to make it look good... and also possible to make it look hideous lol.
Just an idea if you want to try some shapes on your car without damaging it. Buy a tube of insulating foam, lay a piece of cloth or something to protect your trunk and spray the foam on it, then cut/sand it into shape. It will give a better idea of what it will looks like.