Elise S2/S3, (poss. fit others?) shower cap, bought several years ago and only used a couple of times, as I had a really heavy duty one I used to keep at work for when I parked it up there.
Velcro straps secure front end around mirrors, with plastic clips to engage in rear roof location points.
Stored this in the boot to take to race meetings and shows, but only needed occasional use, no rips or tares and not affected by U/V from sunshine!
Nice waterproof canvas backed material, only taken out of bag since I last used it, for pics today.