What's needed is an appropriate shim material that's approximately 1/16 (.063) inch thick. Unless you can find a two-piece metal shim made for this purpose, some sort of flexible plastic or slightly thicker rubber may work.
Personally, that's why I use braze-on FDs with a clamp-on adapter. The clamp-on adapters are cheap ($10 for Shimano or $20 for Campy).
I ended up shimming a Record FD with one of those little rubber strips that comes with the handlebar mount for a cyclocomputer. And it works just fine. Would've preferred to use a clamp with a braze-on but I got into a mix-up with the parts guy (he assured me he could get me a shim) and by the time the mess came to light, the bike was built. Sometimes necessity is the mother of invention.
I'd love to find a braze on adapter that will fit a 28.6mm seat tube without shims. Branford bike sells a shim but it's way overpriced, plus I just like the idea of having the correct fitting part for the job. My bike came with a Mirage clamp-on FD of the correct size. That seems to be the only clamp Campy makes in 28.6mm.
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