I built one from a window screen kit from Lowes. Its lightweight, easy to mount with simple hinges, and has lasted 4+ years with only a couple of "uh oh" moments where I had to bend it back in shape a little. It secures with magnetic strips taped on the opening and top of the frame/door way. I wouldn't call it sturdy, but as long as nobody runs into it or hangs on it, its good to go. Opens and closes with minimal effort. Definitely keeps all the bugs out.

For the rear barn door screen, I purchased a frameless hanging screen and secure it all around with velcro, but have found I seldom use it. Don't even remember using it at all last season.