Looking good.
Centering rings can move freely inside the airframe without any negative consequences (within reason). When you glue them, just make sure that you get a good bead of glue (TiteBond II or Epoxy - better for filling gaps as it doesn't shrink while curing) around the entire circumference of the contact area.
If the centering rings are loose around the motor mount (MMT), that is also not a big deal. That does make it slightly trickier to glue them on perfectly perpendicular to the length of the MMT. As an aid, consider building a masking tape "barrier" on one side of the MMT, place the centering ring with glue on top of it, and let it dry. Then remove the tape, and apply the glue fillet on the side where the tape prevented fillet formation.
Before the glue fully dries, roll the motor mount with the centering rings on the work table, and see if the "wheels" of your "axle" are wobbling too much. If they do, adjust the angle to remove the wobble (use masking tape to keep them in position), and the rings will be perfectly aligned afterwards.
It sounds like your MMT tube is a bit on the thin side of things, thus likely causing a bit of play with the motor retainer and the centering rings.
Another possibility is that you had ordered Aeropack retainer (good choice, BTW) sized for fiberglass tubes, and not the thinner paper ones (they come in two varieties, with slightly different inner diameters).
Again, not a big deal. Use a highly viscous epoxy, like JB Weld (available everywhere, including Walmart). Mix it, apply liberally to the MMT, and for extra brownie points, see to it that it abuts the aft centering ring. Leave it to cure overnight. Done.
https://www.amazon.com/J-B-Weld-826...ocphy=9003763&hvtargid=pla-320192516391&psc=1
HTH,
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