Hi Keleher!
I'm sorry your question has not been answered. If you have not yet solved the problem, I have a suggestion for your consideration:
Might it be possible to use an end-mill or even a common hole saw, and re-mill the hole in the shim off-center? I realise the hole in the shim needs to be larger, but not by much -- about the thickness of the difference between the center bulge and the handlebars. If the shim could be securely clamped and the end-mill cut no closer to the thin end of the shim, there appears to be plenty of stock to enlarge the opening toward the thick end. This would avoid clamping the shifter directly to the steel handlebar.
Clamping the item securely for machining would be the bigger challenge, but I've had great success in re-milling MTB-sized components (shifter pods, re-forged and re-contoured brake levers) to fit on road bike-sized bullhorn handlebars, and this would be essentially the same operation, made easier by the parallel flat sides of the shim.
If you employ anything as a wrap-shim, then the shifter will be centered on the 'bar, just as if you were to clamp it directly. However, if you wish to try --and if there is a rubber-supply store in your town -- I've had great luck making dense, non-slip shims from rubber/belting laminates. They come in differnet thicknesses to suit your needs. Just cut a section long enough to completely surround the bar when butted end-to-end, secure with a short piece of tape (to hold it temporarily in place), and then clamp whatever around it. The friction provided by the rubber means most clamped components (including road bike brake levers on MTB-diamater 'bars) need little clamping force to hold securely.
I hope you've solved the problem already, but these are the best suggestions I can offer at present.
Best,
Dan.