i used to make my own molds with some kind of silicon moulding rubber from Dow (before bhopal) -
relatively expensive, but does the job - especially if you have complicated shapes, want to make several copies, etc. -
you mix it up, paint a thin layer on one side of your artifact, then pour it over slowly, making sure you get the bubbles out -
then flip it over & do the other side -
use some vaseline or something to keep the halves from sticking -
i used to cast with the epoxy from auto-supply stores, meant to go over fiberglass when fixing body parts: you can mix some oil paint in to get a close match to the original colour -
helps if you know about how to make/design moulds; play around with the resin mixture: too much oil paint will make the end product soft, too little & it'll be somewhat transparent...