I actually have 2 covers for my Camaro. There are a few times I had to leave it outside. I have one real soft cotton one and one that is made of Kevlar (I believe that is how you say it.)
I got the cotton one from California Car Cover Company.
http://www.calcarcover.com/how_to_select.aspx . Fits the car like a glove. Even as detailed as to asking me if it had a spoiler or not.
The Kevlar one is loose fitting. I really don't remember where I got it from. I would put the soft one on, then the waterproof one on top of that. One bad thing about car covers is if you use tire shine it will get on the cover. I always make sure I get someone to help me and grab one end of it and shake it real good before I put it on so there won't be any dust on it and I never put it on the car if it was dusty. I was always afraid that would grind the dust into the paint. A poorly used and poor quality car cover is probably worse than not using one at all.