Sulphur is pretty good, but if you overheat it surprisingly little, it will ignite with a low blue flame. There is no risk of a great conflagration, but it gives off sulphur dioxide gas which rusts any steel on which it condenses. It is also quite unpleasant stuff, which in the First World War was found be more dangerous than some gases with a much worse reputation.
Car body filler also makes a good cast. You would probably need to put it in a disposable plastic tube and squirt it into the chamber with a dowel. The combination of weight and fluidity of Cerrosafe (and bullet casting alloys) determines how well the will take the shape of a mould or chamber.