You could probably find either a mill file or a square jeweler's file near or a little over the width of the sight post and sandwich it between two tongue depressors in a vice so that the edges of the tongue depressors stuck out an eighth of an inch or so above the filing edge to keep the post on track while stroking the sight toward the file handle. You'd still have to keep the inverted sight base parallel to the vice jaws by feel, but it is doable.