Just get a piece of glass, or lexan to fit directly into the rubber weatherstrip. Automotive safety glass would work. I don't know the thickness of the fiberglass at the door window opening, but assuming it is 3/16" thick as at the side windows
, and if you use 1/4" thick glass, then the AS1456 weatherstrip would work good as one groove is 3/16 and the other is 1/4". Or you could get a custom window made to fit. Look on eBay for RV windows or teardrop trailer windows, as there are always cheap windows there. You might get lucky and find one that will fit.