I use the clear plastic sheet often available among the dropcloths in the paint aisle of the builders supply stores. I believe it is polyethylene, but test a sample to be sure the stuff you find works. Epoxy does not adhere to the film when I'm laminating, scarfing, etc. Depending on the thickness, you may have difficulty making the sheet conform to complex shapes, but it should work just fine behind a rub rail.

Plastic milk jugs are the same stuff, and hardened epoxy pops out easily. They're good, cheap glue pots.