I found the old overhead projector blanks to be very good. The material has a sheen like glass, is very thin, does not break and it easy to cut. It is impervious to most solvents also which makes if hard to glue but a contact cement like walthers goo works.
I used get used ones from work and cut the clear spots out. Not sure where or whether this material is still available. I have a few sheets left that I bought also.
