George, have you tried black sheet styrene? My other hobby is model railroading and I hand cut my Bahtinov masks from a 1mm thick 8"x16" sheet of black Evergreen sheet styrene(otherwise used to make roofs for model buildings). Styrene is pretty stiff and is readily available at a good hobby shop and the black styrene eliminates the need for painting.
I imagine sizes larger than 8" would be available.