Never heard nor seen such a cover, but I have being giving serious thought to designing and fabricating such a cover for the vent to prevent wasp, spiders, and other flying insects that build net in and around the refrig components. Such a cover would have to be sealed with some foam weather stripping to prevent these little critters from gaining access to this area of the RV. I have some ideas in mind for attachment that would make it easy for installation and removal. The proto-type is on the drawing board at this time, hope to have something for viewing in the next few weeks.
Jim