I have, over the years, collected quite the seemingly odd collection of hard plastic circles: old "new" plastic pill bottles of every size, the caps from large fluid containers (like laundry detergent/softeners etc) and anything else that might prove useful for restoring even, concentric gaps between two pieces of metal.

At times (they seldom work right off the shelf) with a little massaging, they have proven indispensable to making this sort of repair successful, especially with tweeters and smaller "mid" and "radio" speakers but have "saved the day" for larger transducers as well.

I have had good luck using hard plastic and nylon like materials due to the fact that they are perfectly round, non-magnetic and the fact that adhesives are reluctant to bond to them very well if at all (a bonus if any adhesive does somehow manage to creep too far) The height of this type of homemade solution helps too when the two pieces need to be brought togther in a controlled fashion (like the rectangular Peerless tweeters from long ago)

As a result, I have made and kept a number of these tools, almost to the point now of clutter. Problem is, with most of them, they are a one off, one time solution and most likely will never be used a second time, a mixed blessing of sorts.