I am not sure you will be able to get a "coin" that goes into your stock. If I were you, I would explore a laser etching place. Get the dimensions of the hole and see if a laser etching place can make you a brass or stainless steel slug that you could get custom etched with your name or a game scene or what ever trips your fancy. Epoxy into place
I am not sure how big the hole is, but you may be able to take a regular coin (old dollar coin or $.50 cent piece) and turn the diameter down to fit the hole. Or enlarge the hole to fit a coin
Laser etch a round, contrasting piece of wood with a game scene, family crest or what ever to glue into the hole.
Have a piece of glass cut to press fit the hole and put a favorite grandson picture under it or your favorite dog and seal it so that moisture does not get under it.
Laser etch or acid etch the backside of a round piece of glass and press into place
You can get clear pour epoxy. Put a picture in the hole and fill with clear epoxy
There are a couple of ideas - Good Luck and Good Shooting
