I'm aware of ~4 options.

1. JTEK barend shifter for drop bars
2. MicroSHIFT brifter again for drop bars
3. Create some kind of stub shaft for your shifter to secure onto. This was common for Rolhoff shifters. 3D Printing might be a good solution here.
4. File and grind till it fits.
   Example from https://www.flickr.com/photos/27209537@N00/5253435428
   
   This would work equally well on flat bars. There's not a lot to come off, but the clamp band is not very thick either.

I'm aware of ~5 options.

1. JTEK barend shifter for drop bars
2. MicroSHIFT brifter again for drop bars
3. Create some kind of stub shaft for your shifter to secure onto. This was common for Rolhoff shifters. 3D Printing might be a good solution here.
   
4. File and grind till it fits.
   Example from https://www.flickr.com/photos/27209537@N00/5253435428
   
   This would work equally well on flat bars. There's not a lot to come off, but the clamp band is not very thick either.
5. If you have a plastic clamp band, consider using a longer M5 bolt and simply spreading the legs apart. If your clamp band is aluminium, it probably won't stretch far, and is likely to tear. This would be bad.

The shifter units are not particularly expensive, compared to the gearbox itself. My personal approach would be to file it, fit it, say "Bother" and then file it again until a good fit is achieved. You'll need patience, perhaps a bench vice, and a selection of round files and abrasives.

Ultimately the shifter takes pressure on the levers so is under a rotational force around the bars. It has to be clamped down enough to not move.