Make a wood frame, seal the wood with epoxy ( 2 coats) and sand it so there are no bumps, and install your lexan with a rubber or silicone gasket trapped between the epoxy sealed wood and the lexan sheet. You can hold this on with a frame of wood or metal which is secured with bolts or screws.

You can make the gasket yourself from a tube of RTV rubber gasket material, or a silicone caulk. put the material in place, cover it with your "window", which has been well greased so the gsket material won;t stick. and lightly weight or clamp the window into place to evenly spread the gasket compound. When the stuff sets up, remove the window, clean off the grease, and reinstall, but tighten it in more so that it compresses the new gasket evenly.

Lexan expands and contracts a bit -- more so than acrylic. You'll want to mount it so that it can do so, and a gasket-based mounting should allow the material to move around when the temperature changes should do the trick.

PGJ