By the time you're building (or buying) the necessary highly esoteric and expensive ultracentrifuge setup I think you would be well outside the realm of "hobbyist" unless someone insists on the most unreasonably pedantic definition for the term.

Unless we're only considering final assembly. Just gotta get that weapons grade fissile material supplier lined up. That might or might not qualify as rich hobbyist territory depending on how high a price tag is permissible.

You don't have to use the ultracentrifuge, though I don't suppose the power plant you would need for a diffusion plant would be much more attainable.