I've seen a lot of people turn out very good looking masks out of cardstock, fun foam, even paper mache. I think there's one or two around that were done in steel. That said, generally the best film-accurate bios are cast from resin pretty much the same way the originals were done. That is, sculpted in clay, then a negative cast made with silicone and/or plaster, and the final product done in casting resin.
Poking around in the huntorials section will net you a vast array of mask construction knowledge.