For a single pour mold, you can even carve it out of wood, like a 2 by 4.
It will smoke, burn the wood (you can blow out the fire), but it will hold
up for a single lead weight making. How heavy a weight are you trying to
make? And about how big?

You could then make the glass cup from the lead part, or even from a block
of foam that you cut to about the same shape.

A friend once got in trouble with his wife by using one of her frying pans
to make lead disks for putting in the seat pan of our old club two place!

One thing to watch out for (may not be this way with the gypsm stuff that
Paul mentioned), but anything that used water in its making, can trap
water inside. Put molten lead in it, and it can be a bad thing.

Happy slug making!

Steve Leonard