I make mine with two different types of leather.
Having heard of a few instances of flints cutting through thin or soft leather and sparking the frizzen, I made a change.
I use a relatively thin, but tough hard veg tan cow leather for the "face" or the part between frizzen and flint, cut to the same shape and slightly larger than the frizzen face, enough larger for the stitching. Then I use a soft leather like buckskin, either real or Tandy style, for the other side as this is stretchy. This is cut to the same size and shape as the other but some extra is left at the bottom to poke a hole in and attach a thong. Once stitched together the flint won't cut the veg tan and the buckskin will stretch over the thickness and shape of the back of the frizzen. I'll also work some mink or neatsfoot oil into that back piece to help repel moisture while hunting and to help it stretch over the thickness of the frizzen.
Seems like a lot for a simple device, but, less complex to do than to write,,wrote,,, and if one has a nice gun,,, why hang scrap junk on it?