Most any 2 part epoxy will do the job. Make sure the coin is clean as is the wood so it adheres well.
Not any different than how most initial shields and ovals are attached to stocks these days.
The addition of a pin or a small screw soft soldered to the back increases the area the glue has to hold on to. That pin doesn't need to fit firmly into the wood itself, just make room for it and the epoxy/glue will fill the void and capture it and the coin and hold on.
Simple soft solder on the pin. I use a small brass nail for the process on initial shields usually, first trimming the head flat again w/a file. More to clear it and clean it for the soldering process than anything. Snip it to length after it's soldered on.
FWIW, Colt Custom Shop in the 70's and 80's used nothing more than a black mastic adhesive to hold the grip medallions on their ivory and MOP grips. The stuff always stayed somewhat workable soft but very tenacious as far as holding the medallions to the grips in the countersink holes.
Kind of like a rubber cement upon removal, you could ball it up and re-use it later to put the medallions back in place and they were there to stay.
No for-ever glue or riveting them in place.
To remove them was by pushing them free from the back of the grip thru the small hole drilled thru the center of the countersink.
Made changing medalions and refinishing damaged grips easy.
