mrthe,Nice carving of your hobo nickel & casting,I think you could save a lot of silicne rubber compound if you just set the nickel on a peace of class and rolled out your clay and made a frame around the nickel and then poured in the rubber compound you could get mileage out of your rubber mold compound.My molds are only 1/8" thick saving a lot of rubber compound,hope this helps.
J.J.