I've done this exact thing on my 73. I found a drill bit just a touch larger than the holes in the lock ring and reamed out two opposing holes (45mm) Then I took a hunk of hardwood, I think I used maple 1x2 if I remember correctly, and drill a hole with the same bit about 3" from the end. That gave me the first hole. Then using a busted bit I ground a point in a piece about 1/2" long and set it in the opposing hole.
Taking the good drill bit and flipping it over,so the smooth part of the bit was in the wood, I put it thru the wood and into the first hole and lowering the wood down till it touched the pint of the sharpened dowel (the busted bit I sharpened and cut off) and gave it a wack to mark the spot. Remove the wood, remove the bit and drill 2nd hole. Take a second bit the same size, slip it into the 2nd hole along with the first bit and Viola-homemade pin spanner. The ring isn't very tight and it will spin off easily if you drill out the punch marks made by the factory.
I know first hand this works, feels good to make a tool. I've also made the finger socket to remove the clutch basket too. That one was a little more involved but I smile every time I see it rolling around the tool box.