I'm prototyping a hardware that involves many thin walls to bend around curves and such, so it needs to be flexible, cheap, and also transparent. It may also be the same plastic used in a production version.
The only example I can think of is the plastic you find around the inside of a new dress shirt collar. (McMaster Carr doesn't seem to have anything like it, except polycarbonate, which seems expensive and overkill; surely I'm thinking of a much cheaper plastic. Maybe PP, but not finding enough info to confirm, or find out where to get some in that form.) I almost went with transparency rolls for transparency projectors, but it's too thin/flexible (5 mils).