Your jig is interesting, but would not work for me. I use a big 6" machinist's vise on a swivel base. I clamp the barrel in leather at the mid point, swivel the vise to allow one end of the barrel to swing out over the floor, and once I've filed that end, I swivel the vise the other way to do the other end. Then I rotate the barrel to the next flat. I do all eight flats. I would not want the barrel to rotate while I was drawfiling. There are times when I need to put more pressure on one side or the other.
Each to his own....