Someone said Lexan a few posts back. GPW? I have been conjuring up an idea for a window with that.
I can do a sketch if needed but basically you make a frame, wood or plastic , to fit inside the foam wall cut out. Mount the lexan on the outside with a spring loaded hinge, something like this ; http://www.homeclick.com/products/sugat ... 8+55058939 , so the window wants to lift up by the action of the spring loaded hinge. On the inside of the frame put your screen on and a small chain or wire cord going through the frame to the lexan to let the window up or to pull it down. Maybe a loop on the end of the cord to secure it to some hooks or nails or something.