I intend to make a flange around the perimeter, make one half of the mould up to that flange, remove flange, and then make the other half.
This is exactly how it's done at the Metropolitan Museum of Art in their reproductive mold making shop. I took two semesters with them when I was in school. It was a graduate course and I cheated to get in it as an underclassman. One of the best classes I ever had in anything.