BT-30 is an old Estes size.
OD: 0.767
ID: 0.725

Not sure where BT-20+ originated.
OD 0.770"
ID 0.744"

eRockets (Semroc) has both.

You can take a regular 20-55 centering ring and sand it until it fits. Otherwise, print out a template and cut your own. I seldom buy centering rings anymore. I usually make them from cardboard or balsa.



.

__________________
I love sanding.