I've been very happy with my Carcoon. Made in the UK. They sell indoor and outdoor versions. Mine is the outdoor. Plugs into the mains and a couple of built-in fans constantly push air through it to prevent moisture build-up. Also has a trickle charger for the battery.
I have the Covercraft Weathershield up here in Canada. I store the car inside during the winter, but during the summer this a great cover. Fits well and protects from dust and rain - completely waterproof.