Here's some ideas: disassemble it and rebuild it as in the old thread by Bridget on rebuilding the side
windows. Next idea, remove it, go to a glass shop and get a piece of automotive safety glass cut to the same size as the outer part of the window frame and install it with the replacement weatherstrip that we are using for the side
windows, the glass thickness should be close to the same thickness as the original window frame. I had glass cut and installed like this for a tractor cab that I made. Of course a solid glass won't open, but the
Uhaul door window doesn't open much. Next idea, Another member here is in contact with an eBay seller that is going to custom make him a side window at a reasonable price. That may be an option. Also look on eBay for a correct size window, sometimes you'll find some listed under teardrop trailers. Just recently on here someone posted a link to a place in California that had used
windows, it may have been Francesca? Does
Scamp have a door window, I was able to use their rear window.