I have one of these "instant-type" screen doors:
http://www.improvementscatalog.com/p...-bug-screen.do
I got mine at my local Walmart for $13. It's a cheap, easy, semi-permanent fix. I shortened it by about 6" with my sewing machine. We hang it from a continuous strip of industrial strength velcro above the inside of the door frame, plus a few small pieces of velcro (on the hinge side of the door frame) to keep it in place. To keep my dog inside the TM, I simply close the bottom of the TM door since the screen is on the inside of the opening. It won't keep a dog in by itself, but luckily my tiny dog is pretty intimidated by it!
Even though it's weighted on the bottom, it's lightweight and folds up small for storage.
Our TM is in storage but I'll take a picture when we have the trailer open in a couple of weeks.
Betty