There are some other threads with lots more info on car covers but I can tell you I went with the Ultraguard Cover from autoanything.com. Here is the link:
http://www.autoanything.com/products...uard_car_cover
It's a little pricey but I love it. The cover is all weather, it breathes well and has a super soft inner lining that won't harm the finish. It's also custom fitted for the 300C. You may not want to pay this much but do not go for the $25 covers you'll see on Ebay.