In case anyone is considering an alternative to the Porsche car cover, I used an Autochic through the winter and it worked very well. We had a vent stuck open in the underground parking lot which allowed a large amount of snow to get in during one of the many storms we had this year. Even with snow piled up on it, it breathed enough to evacuate the water and leave the paint damage free. It cost US$155 including shipping.

Here are some pics from when I was taking it out of storage:

 

I have to second that AutoChic made me a custom Jersey cotton cover for about $135 shipped to Hawaii, and it has done a great job.