you can get the replacement from 2Pure for the joplin as the internals are the same just the outer that is different .. I have both the 30.8 and the 31.6 posts but only one joplin … sadly I need the same size as you but £28 I think for other size.
Honestly I would try tape , rubber shim whatever to clamp up and think it will be ok but am clearly in a minority here.
Again why not use a rubber shim what is the problem here?
Used for seatpost mounted racks, baby cycle carriers (and at the mounting point), shims for lights etc
Could even try a coke can shim (Zen and the art of Motorcycle maintenance moment)