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.
Comes in various sizes, though the 4-7/8" is the most popular.
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.