A foray into the parts heap produced a copper bolt threaded 1/2″-20 and a matching steel nut. I bandsawed the nut in half, doing a surprisingly good job of cutting it parallel to the surfaces, and filed off the obvious blems. The thin washer fit a 7/16″ bolt until I filed the hole out; the OD is a bit undersized for a 1/2″ head and looks much better in this application.
I grabbed the bolt threads in the lathe and turned down the head for a slip fit in the EMT. Turns out the head wasn’t exactly concentric with the threads, but now the rounded-off hexagon tips are. Drill out the middle for a slip fit around the 11/32″ brass tubing, break the edges, and it’s all good.
The bolt threads need to be barely long enough to go through the aluminum box I’ll eventually mount this thing in and pass through the nut, so I sawed the bolt off to 3/8″, more or less, and cleaned up the end in the lathe.
I thought about soldering the bolt to the EMT shell, but, fortunately, came to my senses before doing any damage. Instead, I drilled & tapped three 4-40 holes in the head that will match with similar clearance holes in the EMT. This is the sort of thing that works really well with “manual” CNC: get the first side lined up, then just type G0 A120 and you’re at the next face. A manual G83 peck drill cycle pokes the hole exactly where it’s needed.
Manual tapping, a bit more edge breaking, some cleanup, and the thing looks pretty good.