Hi Danny,
I don't know why I didn't think about this before. There is a company not far from where I live, they produce things from plastic plates, e.g. chemical tanks and baths etc. They allow me to take cutoffs, free of charge, from their waste containers, this kind of material would be very suitable for the Hi-Vis mast plates.
Please don't ask me what type of material it is, I can't remember, they list the materials they use as PE. PP, PVC, PETG, PVDF. This stuff can be easily cut, filed, and polished, and is available in various thicknesses.
Bye the way the plate for my Hi-Vis mast is from pexiglas, but this is harder to work with than this other stuff.
I don't think you'll find anything cheaper than free.