I am not much of an engraver, the work on you hobo nickel is fantastic. I am a jeweler with 35 years experience. I do stone setting CAD and mold making. I have found this material to be great for a quick simple mold
http://www.riogrande.com/Product/Rio...z/701042?pos=2
For a simple fast mold it works great just kneed the 2 parts together like clay. Lay your coin on a clean flat suface press it on and about 5 minutes later you have a mold with 0% shrinkage..... FWIW