Well, like many others, I have looked at several of the wonderful buildalongs here, and pick up bits of info, and techniques, to use. One of them was to use the drill press as a vertical mill, and put a milling bit on it. Then, laying the riser on its side, you slide it underneath the milling bit and take a bit at a time. This keeps the sight window perfectly parallel to the side of the riser, and allows the transition, on the bottom of the window, to be done with a radius, rather than a sharp angle. Usually, I only use this for the finishing passes, and cut the bulk of the material with a handsaw or a chisel. That`s only one way to do it, there are several...I`ll try to find the source of this technique to post here.
DF