Have you looked into that stuff they sell at the hardware stores that is used to frost household windows? I’ve seen both thin plastic film coatings that are applied like the LCD screen protectors and thicker plastic rigid sheets.

Addendum: Another thing that might be readily available at your local hardware store are the plastic diffusers for light fixtures (i.e., fluorescent light lenses, etc.). There are many styles of these, of course, and some may work better then others for flashlight diffuser function.

P.S.: I have always found the hardware store a great place to wander when looking for improvisation material for a project.