A great idea
OK, for the heater I got the 2 little Camco wire domes that clip on the 2 openings with a spring and a clip, those have worked great on last 2 trailers we've had and I thought they'd blow off.
For the bumper, I also got the Camco '40325' replacement end caps that are just press in fit, press in and pull out and have molded screen type openings and are currently (06/28) $6.37 a set of 2 on Amazon. Those have been great, easier off and on as needed and no more bugs inside the bumper tube - period!
I also used fiberglass screen on the fridge openings, just some silicon to hold the strips on.
I haven't gotten the water heater done yet because as mentioned in this thread I also thought the screen would be to tight for good airflow. I also used the fiberglass screen door screen on the bottom of the indoor floor heater vents to keep small stuff from falling into the floor channel through the vents. That has worked great, no decrease in airflow with the screen on the 4 heater vents
Just cut it a bit large, use some metal duct tape to help hold it on the floor vent itself then the pressure of the vent holds the screen in place when screwed back down. All great ideas!