This is what I use. It doesn't fit perfectly but it has stayed on for over a year now.
Camco 42150 Flying Insect Screen - WH 200,4.5" X 4.5" X 1.3" , white
https://www.amazon.com/dp/B00480BVXS...ing=UTF8&psc=1
We live in Georgia and have been all over the SE was well as out to Glacier last year. Probably towed 15k miles last year.
It has sturdy and strong springs. Never been a problem.
This is what I use for the fridge too.. in case you need it.
Camco 42148 Flying Insect Screen...
https://www.amazon.com/dp/B00192HPOC...p_mob_ap_share
The water heater has a screen too but I used to be from an old camper and dont remember where it came from.