Hi Mustafa, you don't need extremely durable / hard to find exotic stuff. A sheet used as impermeable protective barrier between the coated paper and negative isn't normally reused for more than 2-3 times at maximum. All you need is a film that is very clear, thin (<= 3mils) but stilll easy to handle and transparent to UV. BoPET is cheap, easy to source and perfect for the job.

Regards,
Loris.