I got an Autotechnica one from KMart, cost me $140, it's made of this silvery stuff and has a soft material underlay, it also has a number of breathing vents at the side so no moisture forms under there(that I've seen). It's totally waterproof, has not leaked a drip.
It's what I put on my Supra when the carpoint is already taken. Keeps the car totally out of the elements, it's a little exxy but worth it.