Jim, I'm going to make a recommendation: if the coin is "raw" (not in any kind of packaging already), stop by your local hobby store (I use Hobby Lobby because it's all that exists around here), and pick up a pack of 2x2 flips. I don't know how big a 20 cent piece is, so compare it to a modern coin, then, when you find the coin it's the same size as/smaller than, purchase flips in that size. Seal it in a flip to keep it safe and keep off further fingerprints, dust, etc. until you can get it certified.