On my last trailer we had Mud Daubers building nests in every opening they could find.
Not this time! One of my first purchases was this Camco 42144 Flying Insect Screen - FUR 500, it fit PERFECTLY without even having to use the springs to hold it in place. As a matter of fact, I'm not sure I can get it back off, it snapped on over the vent.
I got it where I get everything, Amazon!