I live in an area that has intense sunlight most days and I have to shield the interior of my car from the heat and harmful UV. The plasticless solution was to simply buy a used bed sheet at a flea market (they are actually called car boot sales here). I drape the sheet over my roof and close the doors on it so it will stay put.
If you have a social obligation to use an actual car cover, please take the advice of a local merchant that I met recently. He told me that the inexpensive covers are not UV resistant and will need to be replaced year after year. I have seen cars on the side streets with these things in tatters. If you feel that you need to get one of these, go for a good quality one that will last longer. They are typically made from Nylon or Polyester.
