You might try two or three cheap dollar store bins. The rectangular ones that are about 2 to 3 inches tall and have square holes in the sides. You could cut both ends off one and one end off of the other two, wire them together to make a cover, then use a piece of small cord or wire to go under the existing ring guard then through the sides of the temporary cover tied over the top. That should leave enough airflow to allow the refrigerator to vent. I wouldn't leave it up there while on the road.
I used round potato salad containers cut down to cover the 30 lb propane cylinders valves to stop birds from nesting inside the collars on top of the cylinders after removing partially built nests. I found the first partial nest after I saw them flying up under the plastic cylinder cover. After removing the partial nests three times I used the potato salad containers.