Why not use beer cans? Cut the top off, punch a hole in the bottom
through which you put your threaded bolt with a nut and washer (to
really hold it in the lead). Put the cans eye-bolt side down into a
pail of sand and pour. Later peel off the aluminum can and Voila.

My company used to be in a building with several other "shady" techie
entrepreneurs. One of them told me he was going to be working on a
"MRI" project so there would be delivery of a huge surplus MRI machine.
When it was delivered, the workers started bringing in large
(10"X10"X2"" very heavy pieces that were painted. Some had holes that
obviously aligned. I noticed the weight and asked what they were and a
worker told me they were "magnets for the MRI, thats why they are so
heavy". I walked over and scratched one with my key and said "magnets
hell, thats lead and thats a gamma ray machine you have there and
unless you have 10X that amount of lead you can stop delivery." The
other guy told me he thought I would be upset by an unlicensed gamma
ray source so he thought he would call it an MRI.
Later, he did obtain a LOT of lead bricks from some nuclear lab, it was
a couple tons and he had it loaded into the bed of his large truck
which he parked on a slight incline. Well, it was an old truck and he
had never bothered to see if its emergency brake would hold so the next
day the truck was found halfway through the wall of our building where
it had rolled. Even more impressive was the sight of so many lead
bricks that had spilled through the cab and collapsed the driver seat.