Here is what I did at our historical society. Mixed media is always a challenge. Select acid free tissue appropriate for silk. Light weight acid free poster-type board. Cotton thread, if you can’t find it use embroidery thread that you unravel. Either side of the medal, poke a hole (remove the medal first) then tie the medal onto the board, padding under the thread with a strip of tissue. I slip the thread under the medal. As needed can pad under the medal to keep it at a proper plane. I then make an envelope of mylar. 3_D objects can be quite fun to figure out best storage methods.