To Jimbo:
These are all fine ideas that boil down to "use centering rings but incorporate them into a removable motor mount assembly." And that probably scratches the itch you initially described, but it doesn't answer the question you originally asked.
So, if you want to have an alternative to centering rings, how about struts? Take straight pieces of plywood or aluminum angle. To one end, attach small plates, curved to fit the outside of the motor mount tube. Glue these to the tube. To the other end attach plates curved to fit the inside of the airframe tube. Glue (or bolt) these into the airframe. Using three or four of these around, repeated in two planes, you've got your motor mount in place with no rings. And if you take the bolt option for attachment to the airframe then it's removable, like the suggestions above, so you've got the best of both.
Keep in mind that if you do this, your booster section is open to the air, so all of your recovery laundry must be above a solid bulkhead, and motor ejection is not an option (but altimeters would love it).
(Hmmm, now I'm thinking of designing a whole booster section out of nothing but struts, like an Erector Set rocket.)