Originally Posted by Random Tandem

Can you fit another washer or two on the nut side, at least just for testing? From your first post I understood that the bolt is tightening some, but not enough to hold the seatpost when you ride - right? If so, it is either the post is too small (not likely if there is no way to get even a small shim in) or the bolt is not tightening (with no seatpost, you should see the gap close, or the bolt is the problem). If the problem is the bolt adding washers at the nut side (where it looks like you already have one washer) should increase the clamping force.

-Will