For that ball, I would go to Jeff Tanner in the UK for a mould. I now have a bunch of his moulds, from .595" to .740" and every one is very nice.
A .790" is .016" larger than 10 bore, btw. A true 12 is .729" to .730".
I personally would slug or measure the bore and groove diameter best I could and get a mould about .010" to .015" smaller than the land to land measurement.