I have one I use on my bikes. The challenge is getting it to stay on while you are moving. Most covers are made for storing your bike while it is stationary. I bought the waterproof one from
HERE. To hold it on I went to the local hardware store and bought some brass grommets. I put the bikes on the rack, and the covers on the bikes, and marked where I wanted the grommet holes. I use 2 small bungees on each bike, one at the bottom of the rim to keep the covers in place. I have yet to loose a cover in 10K plus miles. If there are decent covers made to stay on from the factory, I have not come across one, maybe someone else has.