bike cover for trailer hitch mount

Hi all,
I'm looking for a cover for my tri bike. I transport it on a Thule t2 hitch mounted rack. It sits behind my SUV and a lot of road grime gets sprayed on it when there is rain or wet roads. I'm looking for a good cover that will protect it from road grime, but will not scratch or scuff the paint. Any one have experience with a good cover? Any recommendations?