Here's the adapter that lets me mount the Bogen flex arm to the macro flash bracet.

It's made from a small piece of 3/4 inch steel bar.

The flex arm has a 3/8 inch threaded hole in it. I drilled the metal bar for a 3/8 threaded hole, then used a thread tap to thread the hole. I then cut the end off a 3/8 inch bolt, filed the cut end down smooth, and soldered it into place on the bar.

I also drilled and tapped a couple of holes to hold a couple of #10 thumbscrews. The thumbscrews need nuts on them so they don't slip through the slot in my flash bracket.