I have built conical shafts from wood, metal and aluminum. The upper end is a circle affair with machined edge; the middle section is the conical section, actually a four sided tapered trapazoid affair; and the bottom is a traditional hollow shaft riding in a tapered bearing. All the pieces are easy to cut; only the machined rim needs some extra effort.
I have the machined rim riding on the traditional two bearing points. One is a roller bearing and the other is a shaft attached to a gear reducer/motor combo.
Boy, trying to describe all of this without a picture can be hard! Mel