Go to any art supply store, and buy a sheet of rubylith large enough to cover your screen. Then, take some cardboard (perhaps from something like a fedex box -- that thickness) and create a slip-over cover, where the front of the slip-over is made of the rubylith film. By creating this sleeve, you can be sure that the film will stay in place even in the wind. And when you remove it and store it flat, the cardboard backing will help protect the rubylith.
Alternatively, 4 pieces of tape will hold the rubylith in place nicely.