That sounds like something you could easily build, using one of the $9.99 ez up screens that I just found online...
cut out one of the zipper sides to fit the inside of your door, add some velcro to the sides to hold it up and voila! instant screen door that you can remove easily, like the uhaul
screen! I think the way we plan to set up our ez up screen shelter, we will need only 3 sides to be solid, then the 4th side that would be against the trailer can be cut, using the section in the middle w/ the zipper to cover the door, and then some how attach the 2 cut sides of the big ez up screen to the camper (velcro maybe?) Sounds easy enough in theory.
I will give it a go, and see how it works out. I already have the screen ordered, and if it looks good, I might go ahead and order a few extras. At that price, if I mess it up the first try, it's got 2 sides with zippers, so I can afford to make mistakes with this project!
Thanks for the ideas!