Regular hubs are getting a bit too easy? Try these half spheres, which similarly train hub pinch strength, but are brutally sloped to increase difficulty and reliance on contact friction strength.

mil-prf-13830b

It is highly recommended that you select the "insert" style base given how big and clunky these things are. This would allow you to insert it into an open-top style loading pin (sold separately here) in order to maintain it upright without having to hold it up with your non-lifting hand.