My dad used to have a single cut mill file that was rectangular, with no tang and both side edges were protected (smooth, no filing). It was 4" x 8" and was specifically made for draw filing. That thing was ideal. Well, he loaned it out at some point in the past and you can guess the rest...
At one time, Brownell's sold them but if they still do, I can't seem to find it.
Anyone have any idea or source where one could be had again?