I ran a light leak test on my Perkeo II last fall and among other things, left the red window cover open and facing direct afternoon sun. After a fifteen minute exposure of a blank frame on Tmax 400, I could find no evidence of any effects. I have heard that with some cameras, it is possible for light to bounce around between the back and the pressure plate inside the camera and get to the front of the film, but in my single test, I saw no problem. Of course, 15 minutes is not eight hours at the beach, I suppose some sort of flap isn't a bad idea.
If you could find a thin, slightly springy opaque material, you could tape a movable flap over the window. Orient it so in normal camera position, the taped side is up, then just lift the other edge to check the window.