Both of these owners used a smoked acrylic material approximately 3/16", max of 1/4". To be honest, I do not recall if it was Lexan or something else. It was easy to drill, if that helps identify the material. Both used mechanical fasteners as well as a silicone sealer. Both used SS flathead machine screws with SS trim washers to hold the windows in place, they did not rely on the sealer/adhesive alone.

I did forget an additional project, that would be me replacing the Bomar hatch lenses in our powerboat last winter. Both of these hatches had a translucent material in the 20" x 20" hatch that was crazed and weakened by the sun. I pulled the hatches and took them home, that material was held in place by a gooey tape in some place, by nothing in others. I removed everything from the aluminum frames and cleaned them with alcohol, and went to the same local supplier the previous guys used. I went in, told him what I planned to do and used his knowledge to select the correct material for me. He also cut it to fit. Total bill was $25. I left the protective paper on both sides of the blanks, used my random orbit sander on the edges to shape them to fit the hatch frames. In my case I went to an automotive supply and got a Permatex product, a black silicone sealer/adhesive. I dry fitted the lenses, then pealed one side of the paper off. I applead the adhesive to the the frame and clamped the frame and lense together with as many C-clamps as I could find. I let the sealer cure for twice the suggested length of time. No mechanical fasteners used. Both hatches turned out perfect and were waterproof for as long as we owned the boat.