an option
On two old (1968 and 1971) Cal boats, I replaced the sliding windows with fixed windows by removing the frames and cutting lexan (or some similar plastic) to fit. I reinstalled with liberal amounts of life caulk between both the frame and lexan, and the frame and boat. Worked fine, did not leak. You have to make sure that the sealant you use is compatible with the window material. Some sealants "out gass" and will not adhere to some plastics.