I've always used some kind of waterproof material, such as awning material, or, the most recent, just plain old Fabricland Nylon for $6
It's attached to 4 corners with industrial velcro
If we don't like the black, the wife can come with me and we'll pick whatever shade of blue we want
Only drawback is if you splash water down the back. In your case it would be a permanent mess but maybe it'll give you some ideas ...