Surly Constrictor 28.6mm
The Constrictor was designed specifically to end slipping seat posts.
The extra-thick clamp utilizes an 8mm stainless bolt and accommodates a rear cantilever brake hanger, if you decide to go that route.
The Constrictor is machined out of aluminum billet and black anodized. It’s available in 30.0mm and 33.1mm sizes.
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.