looks like polyethylene film works OK if it is not too thick - need to strike a balance between transmission and protection though. If it is thin, will have no optical aberration effect at these wavelengths
http://www.google.com.au/imgres?imgu...9QEwAg&dur=265
has to be the right material though - other plastics can be opaque, even in thin films - I think the current glad wrap is polyethylene.
http://journals.ametsoc.org/doi/pdf/...F%3E2.0.CO%3B2
to improve mechanical strength over a large area, could use flywire to support the film and put up with some additional loss due to shading effect.