I looked at the seals/nilos rings as a completely separate issue re: spindle.
Ie a separate component holding/positioning them re. shaft nuts, collars, etc.
Kind of like a cover, over the spindle, from which the spindle protrudes, and is then sealed by them.
Thus the cover does not really need any precision, any strength, or any mechanical properties, or any precision, for the tiny-force Nilos rings.
The mechanical solution is bolts, to spindle housing face ... but I think/suggest any suitable locktite-stuff of low strength is probably better.
Probably, even any diy glue of low strength is ideal, easy to remove with hot-air gun, cheap.
Any collar, cone, taper, shaft surface can be used to align stuff, well enough, imho.
After all, You are probably never going to remove the cover, in your lifetime ... and if You do, or someone else needs to, they can always align stuff in a 4-jaw well enough.
As a reductio ad absurdum, a 1 mm thermoplastic, hot-air gun, hand pressure, would likely be fine, as a cover, and the rings slipped on after, glued in, lapped in.
(maybe a temp washer on the spindle, to create a tiny air-gap. Anything, really, from a 40 mm washer to a felt ring - grin).
These type of things have lowered hdd costs, machine tool costs, appliance costs, electronics costs, etc. 100-200x in 15 years.
Everything is small, light, cheap, plastic .. lasts forever .. but is hard to repair/fix/unmount.
Maybe .. If you want to have access to the nut in front, make a larger collar, 2 mm thick, in any metal of you choice..
Use the (bigger) nilos rings to seal to that.
Allows unmount options later.
I would prefer zero nuts at front, drawn in from back via screws, bolts, bellevilles/springs/whatever.