I've found this cheap carbon post on offer which unfortunately is 29,4mm diameter. The seattube on my Principia aluminium bike is 31,6mm. I have'nt been able to find any shim that to fit these exact dimensions, but a 29.6 to 31,8 aluminum shim comes pretty close... The shop says this shim should work. What do you think? As a carbon post is rather fragile I want to make sure that I can both get the shim in the seattube, but also get a secure grip and does'nt damage anything.