If you have a train store around, look at the scratch supplies for clear sheets. They are usually right near the regular sheets of plastic. You probably already know that, I just wanted to make sure.
https://plastruct.com/product-...d-sheet/clear-sheet/
https://evergreenscalemodels.c...ystrene-clear-sheets
I'm also tempted to try the plastic from carded things that gets thrown away anyways. Sometimes it feels thicker and might be a suitable replacement?
Just glue with hot melt or rubber cement. Superglue makes it go white.