I just got this email from my local Toyota dealer, selling a car roof cover. I think it's pretty cool, it's half a cover so it's easy to put on and remove. Can keep your car and battery cooler... http://pictures.dealer.com/l/longogroup/0490/49efd33beab9b003441d8318e7f895a0x.jpg
This is really cool. One size fit all? On the other hand, I would prefer something more hi-tech like this: