It you get a 3/16" milling cutter, and an angle plate, set the angle plate up on the cross slide, at 45°, then set the work up on the angle plate, at the desired angle for the tool to "lean in" to the work. Then mill the slot, the cutter will mill out oversize anyway and you'll be pretty close to 1/4" while leaving room for clamping up. Put another piece of stock on the angle plate without disturbing the angle, a piece of stock bolted under the piece you just cut can act as a jig, and mill a slot in another piece of steel. Either in a drill press, or the faceplate, or in a chuck. Drill the hole for the mounting screw in both pieces of work, align them with a 1/4" piece of HSS, and clamp together, in hindsight it's probably easier to do this with the faceplate parralel to the chuck body and clamped. Tap through for a clamping screw, then drill out the smaller top piece for clearance.
All ready to go, and the relief that is shown on John B's plan can be filed ground or milled to finish the job proper.
All done without a mill.

