This site
Cal Car Covers will help you select a fabric that suites your weather conditions.
I used a Noah fabric cover from them to protect from bird drippings, snow and ice. No problem over 4 years use. Good as new. And I'd take a push broom to get the 8" of snow off and up to 1" of ice to take it out occasionally when we got a break in the temperatures. Even under melting snow I always found the car itself dry.
You want something that is soft on the inside so as not to abrade the car's finish, a clean car so dirt doesn't get ground down and a tight fit so the cover doesn't get pulled around in the wind. Also something that can be washed in a commercial laundry.
For a Porsche, one man fitment was trivial. For a larger machine, no idea.