A planetoid with tunnels, caverns, farms and factories, hospitals and academies; all safely beneath the surface. A world unto itself.
A very tightly controlled society broken into for or so exogamous clans but all subject, of necessity, to restrictions on childbearing and employment.
Optimum would be to take a metallic asteroid, or other mass, and manipulate it- like a glassblower- into a cylinder.
Boy I pity the first engineers in charge of that project!