I’ll start by saying I have not made a rum horn yet, but have some blanks set aside for it someday. I would start with a good white horn of at least medium thickness or more - to better hold the stopper, as the top pour hole is a weak point in the horn body. The nicer examples I’ve seen (contemporary and original) have a raised oval, diamond, or square-shaped escutcheon left high around the top pour hole for support of the stopper, and the rest of the horn body taken down around it to lighten the weight, and to help preserve the hole integrity over time.