I've a 27.2mm seatpost that gradually slips down into the seat tube despite the clamp being tightened all the way to the torque limit of the clamp bolt. Obviously, the bike manufacturer had poor manufacturing tolerances on my particular frame.
The steel seat-tube shims that the bike shop sells are MUCH too thick to work. The mechanic suggested making a semi-circular shim from a coke can with a pair of tin snips. This seems doable.
My question is, how do I form a "lip" at the top of the shim to prevent it from sliding down into the seat tube? My thought was to position in in the frame & use a small flat-tip screwdriver as a chisel point to bend a part of the shim over the top of the seat tube. Is there an easier option?