EXACTLY!!!Cook the plaster mold in your oven for a while to drive all the water out of it.
Melt some fishing weights and pour the liquid into the mold.
Let it cool.
Break it out.
File to fit.
Drill a hole in it.
it is fun and not that hard.
off-topic
we actually make our own fishing weights with special 2 piece mold. the usual waters we fish in are full of junk at the bottom, so when reeling the line in it is not a good idea to drag our contraption at the bottom as it might catch an old branch. our weights are shaped as self aligning airfoil, when dragged in water they are forced upwards puling our equipment out, clearing any bottom junk. as result we loose significantly less tackle
since our tackle is also custom made this means less time to tinker with it (or build repalcement), more time to fish 
