Relax, Machined seat post sizing Shims work very well ,
I now have 3 bikes , over sized seat tubes, with sizing shims
to use common 27.2 seatposts , 2 are cane creek Thudbuster seat posts..

in fact one of them, 3rd example; a Bike Friday Tikit,
I took out the 27.2 Kalloy Uno
seat post, head slipped angle too easily..
and reused my 25.0 <C> SR* with a 25>27.2 USE seat post shim,
so its a Sleeve inside a sleeve, inside the seat tube.

* came out of my screwed & Glued AlAn..