The stock bar mount for my Sportster was for a two-piece bar. Separate lefts and rights.
Left side has timing advance, right has throttle. Both internal. The mount is cast steel and in only bored in from each end just long enough to grip the spline on the end of the bars. For using a one piece bar you need to get post 74 parts. Or get creative.
I cut a chunk of handlebar and chucked it in the lathe. Clamped the bar mount on it, tweaked the other end around until both ends were as concentric as possible. Bore up to roughly center to match old bore. The old bore was a little big, that's why the ends aren't quite cleaned up.After boring to center flip it around on the chunk of handlebar and reset it. Bore to meet the other bore you just finished. Fingers crossed the bores meet in the middle.
I made a quick fixture to hold the part in the vise. Just a chunk of aluminum that I drilled two holes in. As ugly as that block of aluminum is, it was flat and parallel within .002". I turned, drilled and tapped a couple bosses for the new bolts. Clamped the part up and scalloped the bar mount mirroring the hole location centerlines of the opposing bolts.
After deburring I blasted it and test fit it. I'm quite happy with how this looks. It's got the same style as the original and I can use whatever bar I want. I tried to paint it but used a ten year old can of paint and it worked about as well as tossing some house paint through a box fan. Needless to say I'll need to blast it again and repaint but the fab work is done on this piece. Now to get that steering damper adapted to this bike (it's not HD but it was free).