As jwfails says, for Lance trailers, Autoweave in Denver makes covers. They are vinyl, and can be ordered with a closed-cell foam backing. They sew snaps into the cover, and supply snaps with adhesive backs to put on your trailer. I have one on my trailer, and have traveled about 8,000 miles with no problems with the snaps. Cover is on when we are on the road, and I remove it when we get to camp.
If you have the dimensions of your window, Autoweave may very well make a cover for you. They have been making covers for the Lance window for a few years, so they have it down very well. I think mine was about $225, but that is with a template they already have. Very-well worth it, in my book. I have replaced my pickup windshield twice since I got our trailer. Not worth the risk to the trailer window, when a cover can likely prevent the breakage.