It's not really a shim that this needs, it's a screw and a specific wedge that presses against an opposing wedge-piece with a half-tubular clamping bit with ridges. A picture would help, but I don't have one
You also might need the plastic clip-in plug for the front that has the "C" winged Cinelli logo...or not, many folks discarded them.
I'd say offer to trade it to a Pinarello owner for a blank un-panto'ed one, that should get a bite.