How are you coin carvers holding the coin while you are working on it? I tried using GRS thermoplastic, but when I tightened it up in my vise the coin flipped out. My plastic holder was made like the GRS demonstration for holding their Buck knives.
Get a hot glue gun and hot-glue it to a small block of wood, 1.5x 1.5 x 0.5 inches. You can hold it in an engraving block that way. Heat up the coin to get it back off, then soak it in coleman lantern fuel to get the rest of the glue off.
I have a piece of delrin machined out and then split part way to hold nickles, clamped in engraving ball. I know a few guys who use the small pins in the top of the vice to hold coins, I think I will use that method for other than nickles. The glue works good also