Throwing your weight around...

[doctorwayne]

I'm not sure how the plasticine would stand up to the hot lead or if moisture within it would cause any "fireworks" - rapidly-formed steam can be very dangerous. If you could make the mould out of something like modelling clay, then bake it hard, you might have something useable, although I doubt that the lead would flow into mostly-enclosed cavities unless venting was provided. The molten lead is not that far above its melting point, so there's not a lot of time that it's still relatively free-flowing.

Wayne