I've not worked with lead, but have done some product designs with plastics on two part molds.
I think I've read everything, but in case this is redundant, my apologies....
1- you have the "female" tin already seasoned/release agent process down. I'm guessing you are inserting an eye or loop while the lead is still molten.
2- having a 2nd part "male" mold that would push down into the lead would give you the concave feature. You'd have to figure how much lip and how wide you want it.....
3-... but I would think a matching muffin pan would work if inserted on top of the lead. This would push the lead up the sides of the female tin receptacles.
4- this leaves drilling. from guys I know that have done their own Carolina rigs, drilling lead is not fun.....
5- could you drill and nail, from the top side, a nail peg in the bottom of each of the tins that is coming in from the top?
When you press the top (male) mold into the female mold, the nails in the bottom would peg the hole. The nails could be epoxyed from the top, and if all were the same length, it would set the depth of your concave shape the same. The variable would be how much lead you put in each of the female tins.