I'm a bit confused by the post. Have you actually measured anything?
I ask because clip-on bars are usually mounted so the clamps are about 6" apart, or roughly 3" from center. That's beyond the clamp bulge, so both 26mm and 31.8mm bars would have already been reduced to the grip/brake clamp diameter of 15/16" (23.8mm), and that's what the clip-on bar clamps would be.
As I said, I'm confused as to why you think they need to be shimmed.
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