Depending on the inner diameter which determines your wall thickness... you don’t want to go too thin... you can always have a machine shop turn it down to 26.8. I’ve done this to several seatposts for my Kilo TT. You can do this by hand as well with high grit sandpaper and a lot of time, or with a bench grinder and a softer wheel which can be had for cheap at harbor freight. I have a lathe and would be happy to turn down a seatpost for you no labor charge, just S&H if you were ever interested.