Rog, don't use Plexiglass because it yellows and scratches easily. For a few bucks more you can pick up a sheet of Lexan, Acrylite, or other brands, cut to size, from your local window glass shop. Those types of plastic are much tougher. Inland RV sells some with an anti-scratch coating, pre-cut to size. Home Depot stocks Lexan, but you'll need to cut it yourself.
Silicone or double-sided tape are fine to secure it in the hinge clamp. When you replace the window you'll want the metal edges found on the later 60s models to hide the cut edge of the plastic. Inland RV sells those, too, or you can try salvage. Secure that to the plastic with a few dabs of silicone as well.