Yeah, I know about the film as well. The only thing I had handy were a couple of floppies though, so I thought I'd try them out first. Unfortunately they didn't block the wavelengths and spurious noise I was hoping for. At least in my limited test. I'm actually looking block visible light and some IR, to filter out some noise produced by some CCFL backlights and Plasma screens (for customers).

I've also got some 5.25" floppies around that I haven't touched in over 20 years. I wonder if they'll still read - I'd love to pull some content off them (Commodore 64 format).