Thisshim will let you easily fit the 27.2mm or 31.6mmDual-Position Seatpost or ShockStop Seatpost into your larger seat tube. Just drop the shim in your frame’s seat tube and insert the seatpost. Once installed, the shim is essentially invisible.

Details:

27.2mm seatpost size can be adapted to 28.6, 29.8, 30.0, 30.4, 30.9, 31.6, and 31.8mm outer diameter (seat tube diameter)
31.6mm seatpost size can be adapted to 34.9mm outer diameter (seat tube diameter)
100mm (4-inch) length spreads out clamping load along seatpost and frame