USE Alien Shim
The USE shim system allows you to fit a seat post into virtually any frame size. Shims are made to 100mm length and are longer than other shims on the market giving your seat post much better support inside the frame. All shims are designed with a "top lip" to stop shim sliding into the frame.
Shims are available in Aluminum and Thermo Plastic, despite the latter being a lighter solution its material characteristics limit the thickness of the ID - OD measurements. Aluminum is used to achieve the thinnest shims as it is a more robust material.