You could also use cement in a can. Thus avoiding the hole melting thing as I don’t know if a thin tin can take that heat. Since one time I put a pop can in a fire to see if it would melt and it just turned black. I then poked it with a stick it turned into ash. So I have no idea.
The lead diving weights are a great idea, as for paint from home depot you could find an epoxy paint which is a rubbery type paint used to waterproof pools and such. Not sure if they sell it there however.
Just my 2 cents, I love to MacGyver things :)