I think you need to re-check your calculations. A 31.8mm clamp is 2mm larger than a 29.8mm clamp, but a shim to work will be 1mm thick, as we are talking about the diameter of clamp, so the shim will be around both sides.
I have a 26.8mm seatpost that I wanted to use with a 27.2mm seat tube. .4mm shim, right? WRONG! I got some .2mm shim stock and made a shim that works perfectly. Again, half the required difference, since both sides are in play.
To use a 34.9mm clamp, the difference is 5.1mm, so shim should be 2.55mm thick. I think you will have better luck finding 1mm shim stock.
Last edited by badamsjr; 07-21-10 at 08:19 PM.