Try “plasticine”, aka “plasticina”, aka Roman Plasticine. Store in a
plastic bag. Needs to be kneaded for full plasticity, but will take
a good impression, it can be “smeared” smooth or totally re-kneaded.
Not sure how it will store in a wood box mold, but certainly
understand why you want to do that. Mine is gray (I think it comes in
different colors, but gray would definitely simulate silver better
than purple, lets say) in color and I bought it from Jewelry/Resource
Supply in Seattle (who now owns Alpha Supply). Not sure whether they
still have it, as it was part of a workshop I took up there last
June, but I bet they could get it for you, or at least tell you where
to get it. Just a note, cold temps make it smearable, but pretty hard
to move as one unit. Got to knead, knead, knead. It likes 98.6
degrees from the hands.
Good luck! (PSMissed you at Wistaria Festival this year–I "lived"
across the street from you in '08)