Bug Screens keep critters out of the openings of the Casita’s appliances. Mud Dauber Wasps are a particularly nasty bug that love to camp in Casitas too. One time I opened the refrigerator access panel and there was a big mud nest on the cooling fins. Luckily the wasps were gone, so it was easy to clear out.
I use screens on the furnace intake/exhaust, hot water heater cover, and the top panel for the fridge. I left the bottom fridge panel screen-less to help with airflow. Critters like warmth, so they’re more likely to go in the upper vents where the cooling fins are.
They install with springs and a little tool that reaches in and hooks them to the grill. It’s a little tedious, but once attached, they’re on pretty well. The fridge covers are held on by small zip ties.
I used the Camco fridge screens in the first link. The Camco water heater and furnace screens will work, but they have lower quality springs to attach them. JCJ makes better, more expensive, “Made in the USA” screens. They’re available from Little House Customs.