I worry about putting anything on my fenders as well. I just can't get behind most fender covers that have a vinyl backing. At some point, dirt is just going to pushed into it and cause scratches...maybe small scratches, but that's not acceptable to me.
I much prefer using a soft towel that can be washed. I hold it in place with a couple magnets and call it good.
1969 Raven Black M Code Mach 1 with 351W
1965 Shelby AC COBRA Wimbleton Green Factory Five MKII
1965 Candy Apple Red Fastback/Convertible