While this idea I came up with is not 100% true to the period of earlier eras, I'ii offer it with the thinking that it may help some with aging eyesight like mine. If you make a simple L of flat metal, as illustrated in other posts, and drill a large hole in the aperture position, the adjustment can be made by super-gluing a washer of the desired aperture size and manipulating it around the interior of the large hole until the desired position for accurate shooting is found. If it is accidentally placed incorrectly, it can be easily removed by the use of a flame and repositioned.