The windows are made by Champion. The size looks to be 1/4 x 15/32. It is very thin and flexible. I bought some that is 9/32 x 1/2 that is much stiffer and does not quite fit.
Bob from SWISCO responded:
April 22, 2019
Thanks for posting. Start by looking at our 55-006. If you want, you can order a sample from our samples page here. Samples are free; only shipping charges apply.