I made my screen door for about $15..
4 hinges
hook and loop to latch it closed
1x3 for the frame
screen
Folable so it takes less room when open, very simple. Works great.
in a lil 5x8 , the bed when its out takes up most of the room so a folding door was the only solution.
The screen door doesnt have to fit exactly, i just cut the screen about 3 inches too big around the outside, then made a big loop so it would suck up all the space around the outside. If you click the pics to enlarge them, you will see.
Also for the winter months, very easy to remove. Just push the pins out of the 2 hinges holding it to the trailer. Simple.
No bugs get in.... hope this helps. If you need anymore pics let me know.