Split shot Mold Question?

I bought a used split shot mold. I didn't want removable, so bought a used one. I assumed it was supposed to have an insert with it, but didn't. What does the insert look like? Can I make one? I used a piece of soda can and it worked for a while, but need something a little better.

Find a piece of metal banding that fits. It comes in 3/8, 1/2, 3/4, 1 inch. I think I used 1/2 in.. Make it longer and drill a hole in it and put a cord through it. Cut a few of them.

Pure lead works better for split shots.

Thanks