I will be replacing 3 windows in a large fiberglass pond I am setting up.I must do this because I flexed the fiberglass during installation and cracked lexan where bolts go through. It will be a diy window install on round pond wall surface ,using 1/16 lexan 1/2 thick starboard frame,and then bolting to fiberglass. the sandwich will be,from inside out - starboard-lexan-silicone-fiberglass-starboard. plenty stiff. should never crack again