Before I had custom screens made for "camper canvas", I bought a Velcro Screen Door Kit. I bought it at local marine store. You cut the screen material to fit and used the Velcro tabs to hold it in place. Not very good going in and out of the gangway, but it was very good at keeping the bugs out so we could have a TV or reading light on after dark.
But the custom screens were best. And skeeters can't fly through the force of a fan's breeze either. Off® Mosquito Lamps work well instead of a screen.......