A quality replacement quick release.
Standard replacement as fitted on the Club, Trainer and Hoppley unicycles.
A 22.2mm seat post uses a 25.4mm clamp.
A 25.4mm seat post uses a 28.6mm clamp.
A 27.2mm seat post uses a 31.8mm clamp.
Some Eclipse and Equinox frames need a 34.9mm clamp
These are a rule of thumb. Yet, variety exists.