I made a crude broach, using the following process. I stuck a piece of aluminum stock in a lathe, and drilled out a hole the same size as was the hex, across the flats (the smaller dimension). Then, I cut off a piece of allen wrench, used a torch to heat up the tip and hit it with a hammer to swell the end. After cooling, I ground it sharp. Then, I put that piece, swelled end out, in the lathe's tailstock chuck and repeatedly rammed it against the tube material. About 5 hits later, it hit depth, making a female cut hex. A crude broach. I torched and hit the cutting tool so that the hex hole would be bigger than the allen wrench that was to mate. I didn't want too tight a fit.