If you decide to try one of the manufactured shims I would suggest that you try the larger of the two first. A too-large post simply can't be inserted, clamping down on a too-small post may cause seat tube damage. The same applies with any homemade shims you might try; make sure that the top of the tube slot does not close past parallel (or the ears if present don't touch ) or you may permanently distort the tube. If you can't quite clamp the post tightly enough some carbon assembly paste may keep the post from sliding down.