Hi Eric,
I've made the short knurled wrench shown at the right in the first picture in the previous post: it is bored to accept the spindle, with two
portions of M4 screws acting as pins to engage the holes into the tightening rings. I've not yet used it, if not on the external
ring.
Some words and pictures to help focusing the problem, there may be a better solution for it.
The first picture shows the spindle, its sleeve and the upper (A, going to D) and lower (B going to C) ring nuts. The upper ring nut has two tapped holes
for M6 SH tightening screws (they should act on the slots and expand the nut).

The second picture shows the complete spindle (put a bit of grease on it) with the ring nuts already inserted: as You can (barely) see, there's no way to reach the upper
ring if not by fitting a screwdriver into one of the holes and hoping for the best (the ring will go a good deal down the sleeve, before
engaging the upper bearing). That's what I did, then locked it into position trusting the lower one to do the job.
It did, and does, for a while: then I start getting play and chatter (the lower nut still tight), open the mill and find the upper one running loose.
Plans for the next time it happens are about sliding my homade key onto the spindle to set the upper nut well tight first, then lock it and use that
same key to tighten the lower nut.

The portion of water pipe I had cut and used with the purpose of pushing the upper bearing into contact with its taper, so it went into
the mill toolbox, where I will find it again at need.
Marcello