Dremel with a stone. I started with a stone (disc shaped) about the diameter of a quarter and maybe 1/4 inch thick. I chucked it up in Dremel turned it on high and went to my bench grinder and fired it up and working the Dremal on the spinning grinder I reduced the diameter to about the size of a dime and tapered the edges giving it a shape like a discus or Frisbee.....kind of a V with a blunted tip. This shape will give you the tapering in the cuts that are noticeable at the ends of the notches. If you use a squared stone then you will not get the tapering at the ends of the cut it will have blunt squared ends.
Then I practiced a couple of cuts on scrap...I put tape in front of area being notched on the actual receiver so that if it ran forward or skipped it would provide some protection...it worked out pretty good. Stones in my experience cut best on highest Dremel speeds.