Use Lexan polycarbonate sheets. I replaced a bottom diffuser in a mixing box of an enlarger that has a 200W 24V bulb with a 2mm Lexan sheet and it passed the several-hour test. It is milky white, though, and very hard and glossy on one side while the other is slightly matt. There are clear versions available and likely free - ask for samples (mine's a sample). They deliver them protected on both sides.