answer:Having no experience in this regard whatsoever, my first thought is that I’d try to turn it on a lathe. You’d have to stop at the contact points, obviously, and finish those by hand, but I think you’d be able to get most of the roundness cut on the turning piece, and you’d only have those small areas to finish with hand tools and a sanding block. EDIT: Thinking about this further, I believe you should be able to cut a round blank out of a piece of dimensioned lumber having the same circle diameter as the thickness of your blank. That is, a roughly 100 mm circle cut from 100 mm x 100 mm lumber (4×4 in the USA). Then you could mount that circle in the lathe, and your finish work would be limited to sanding and covering up those points of lathe contact.