8 Pcs Adjustable Hand Reamer Set H4 to H11-15/32" to 1.1/16" Reaming Tool for Precise Hole Finishing with 6 Carbon Steel Blades- Hand Reamers for Metal & Wood - 11.90mm to 27mm. - Wooden Boxed.

The risk-free way to do it would be to buy a set of adjustable hand reamers that will go up to 26.2mm and a big tap wrench to turn them. You then start small and slowly ream the entire seatpost away. It will take forever as you're able to cut about 0.2mm max per pass, then have to with draw the reamer, expand it, and go again... But it's self aligning, and very low impact. Pro-tip if you do this: only ever turn a reamer in the cut direction even when withdrawing it, turning it backwards blunts them very quickly.