Take an old file and on it mark out an octagonal shape measured from the muzzle of the barrel and reduced for the smallest distance across the barrel flats. Masking tape works well for the lay out. Cut to size (slow grind so as not to kill the temper) sharpen the edges and put a handle on the file spur. Bend the spur or tang to about a 120 degree angle. Work slow and w/o too much pressure. Best to work for taper back and taper forward so as not to open center of channel too much.
