The refrigerator vents are an entry point for rodents and mud-dauber wasps. Camco makes a mud-dauber screen for various sizes of refrigerator vent, but it’s not effective for rodents. One of my vent covers had some plastic gnawed out to enlarge the opening.
Here is the Camco mud-dauber screen installed.
Rodents can get through very narrow openings because they don’t have collar bones. Anything their skull gets through, the rest will get through as well. I have used 1/4 inch mesh, if this fails I’ll have to get 1/8.
You will need gloves and for this work. Cut steel screening material has sharp ends that will draw blood from your hands. Use a tin-snip (a scissors for sheet metal) to cut the screening.
Form the mesh to fit the inside of the refrigerator vent, and bind the corners together with steel wire. The screen came with several feet of wire.
Shape the screening to tightly fit the protruding portion of the vent, or the vent won’t fit in it’s opening.