I have found that regular 12 gague electric romex will work for that size pin. I have a bunch cut and if they slide out of the jig mold then put a small piece of duct tape over one side of the pin slot. That will keep them in place.
With them you can pour, paint, then pull them out. They cost less than 1/2 a cent each. 10 gauge may be better but I don't have any of that laying around.