Being a bullet caster, and with lead getting scarce just about everywhere, here is something I tried last year, and it worked pretty well.
I had some left over concrete (quikcrete) from a patio project, so I took some small cans (from snack pacs) and filled them with concrete then placed a piece of wire in the concrete and let it set....works well, albeit a little big. I've since used the leftover concrete to make weights for my gang rigs using various size tin cans, styrofoam cups, plastic cups, ect.
After the concrete is thoroughly dry, I take and give it a heavy coating of latex paint (you can usually find bargain paint at any home improvement store, usually in drab colors, for a bargain....just ask the manager. People bring it back because it's not the right color, ect). The paint helps protect the concrete as well as protecting the finish on the decoys as well.
FWIW