As original parts & 27.2, post ,bike friday, Thud buster post, stock generic binder
gave way to a Surly/QBP 'constrictor' .. but still slipped.. up near stripping threads tight
so I got one of the Ahrens 'wise crackers' in 27.2, and clamped it directly on the seat post itself.

and the combination worked..

did something Similar on my Brompton, though the band directly around the seatpost,
was a de-lipped QR one of, appropriately, larger diameter,
and to fold the bike , both QR levers get opened.