Only 26.8mm will fit.

Make sure that the 26.8mm you measured is accurate. Does it matched any engraved numbering on the post?
Also measure it at 3-different heights on the post.
And measure at the sides and at front/rear positions.
Total number of measurements 3 heights * 2 orientations = 6 measurements
Should be all 26.8mm. Otherwise take the area of largest measurement and see if it will slide down into the seat-tube.