Ray:
I use 0.010" thick polycarbonate that I bought in a roll from TAP Plastics. It really looks like glass when installed in a model. Polycarbonate (GE trademarked as “Lexan”) is very clear, highly UV resistant, unbreakable and is actually used as passenger car windows in areas where vandals continuously throw rocks that break glass windows.
The 0.010" thickness scales to less than 1/4" in 1:20 scale, so it closely approximates window glass. I install it with a Testors cement that is designed to be used on plastic canopies in model airplanes. It does a good job, even on installs in wood frames. The roll material comes with protector sheets on both sides, so it won’t get scratched in transit or storage.
Many folks use pieces of plastic from various plastic containers such as disc boxes, but that stuff always becomes brittle or changes color in ht e sun. The microscope slides I used in smaller scales are breakable and sometimes too small to do a whole window in LS.
I see there is no TAP Plastics store in S.D., but TAP will ship anywhere. Russ Miller is the store manager at the San Leandro, CA store and is also a serious LS modeler. He is really helpful in getting the right stuff to us modelers who live further afield. If you can’t find this material elsewhere, I suggest giving Russ a call.
Happy RRing,
Jerry Bowers