I had thought you were all done with this, Bobby.
I did this back a few months. I used another similar sized hex that I rounded the corners off a fair bit, for the first form. I alternately tapped the Sankey tube above each flat with a hammer, and smacked the tube down over the hex from the opposite end with a spiral wound leather mallet. When it was close to the correct shape, I then used an actual compression nut that I only slightly removed the hex corners on, to finish the hex shape in the SS tube.
I first made sure of correct tightness for each sight tube for each keg, and marked each tube for that keg. After getting the tubes to sit down to a uniform depth on all three keggles, I assigned each tube to a keggle, for the orientation of the slots for each keggle. I marked each tube for its correct direction outwards, so that the slot would appear correctly aligned, to the clocking of the hex flats for it's associated compression hex nut. I then just used nails driven part way into a wood bench to hold the tube. I put a set of nails on either side of the tube, a 1/4 of the length in from each end, and bent them inwards to hold the tube side to side, and put a nail at each end to keep it form sliding towards or away form me.
I used a heavy duty angle grinder and coarse stone-type wheel to coarse grind the tube to a Sharpie outline I had drawn when it was installed over the glass tube on the keg. When I had most of the stock removed, I then used a 5" air grinder with a 120 grit disk to shape the sides of the slot parallel and to size. I checked the width of the slot with a set of calipers to do the parallelism, but just eyed the straightness of the slot. The ends are formed to shallow out as to make an oval shape start at each end, rather than a square-ish shape as you have shown. A little work with an air die grinder to de-burr and fine finish the shape, and I was done.
I just inserted the next tube into the nails to start coarse grinding it. The SS tubes get VERY hot during grinding, and the removal is poor; it mostly bends under and must be ground away after getting thin, by the 5" disk grinder during the coarse grinding, then continuing with the angle grinder once again.
I cut the tubes to the length required by my placement of the SS eyebolt at the top of the upper skirt. I cut them off square with a thin cutoff (1/16") wheel on a die grinder, and after carefully de-burring everything with a burr knife ( a sharpened 3-corner-file type of deburring tool), and a fine round file.
I also used a set of o-rings from Corny posts to hold tube central to eye bolt, which had the effect of holding the SS tube central to the glass tube. The same o-rings were also used as a sort of compression ferule inside the 1/2"NPTx1/2"tube 90* compression fittings. I did not use the SS ferules that came with the fitting.
I take no credit for any of the concept, it was all Bobby's idea and I also got the glass tubes from Bobby. My contribution to this is my execution of the concept.