Tom I'm sure the OP doesn't consider this a "trivial" operation to his experience & machining history...It might be for you and I, but he's struggling a bit here.
D, from your earlier posts, I would say that "layering" a riser would be easier, cheaper and you can do it yourself, gaining a bit of "experience" while doing . Layering allows you to use much more attainable material and cheaper because you don't have to "farm out" the machining because your machines are too small to do on the single large piece of bar.
I have built many a piece for a jig or two that required 4-6 layers of 1/2 or 3/4 flat bar. Of course a certain amount of "extra work is involved machining the pieces flat to join together and a lot of drilling & taping for joining bolts...but its easily doable with rudimentary hobby machines. In your case I wouldn't even consider that the finished piece be round...esthetically speaking, it would be nice but the reality of the clearance around the column provides for the odd square corner to not interfere at all. flat bar stacked together would give you what is needed.
Dabblers idea would work excellent as is but for my peace of mind I would also weld a gusset from top plate to bottom plate at the outer edges for a bit of "over engineering" that I am often accused of . Just a piece of 1/2" flat bar welded off each flat of the square tubing out to the outer edge so as to not interfere with the bolts will suffice.
I am wondering, are those studs in the mill base or are they bolts that have a head on them hidden in the square part of the base? if they are studs, they might bring up another problem to the "single piece of hard bar for you...proper metric studs of the proper length might be a search issue adding to the issue.