I once cast lead in a clay mold.
What kind of clay? Oil-based clay melts when heated.
I can imagine that water-based clay could crack and spill (or even by lever-action,
launch) hot lead around. I know from experience that plaster is very likely to crack if there is any tiny bit of moisture left in it.
Heat-resistant molding silicone is the right stuff to use IMO. It is not that expensive, really., and it is readily available on-line or from craft stores. Anyone can use it. The most tricky bit is to measure the right number of drops of hardener. If you add "silicone thickener", then you don't even have to build a mother-mold, you could just brush the silicone onto what it is that you want to mold. There are lots of tutorials on-line to help you get started. There is also non-heat resistant molding silicone, food-grade silicone, medical grade silicone etc. but they all work the same way, so most info that is out there will be useful for all types.