"Tom Gardner" wrote: (clip) "You learn that file, hacksaw and welding and
you'll KNOW machining."
^^^^^^^^^^^^^^^^^^
I think Tom has the right idea. But I'll be more specific. Hack saw or
band saw the round bar to the required depth. Heat it red hot with your
acetylene torch, and hammer it around the square piece that you want it to
fit. Drill a hole to accept the clamp screw. If necessary, turn the
outside round in one of your many lathes.