I have some 4 inch BT, namely some Estes BT-101. Is that close enough?
Other options for tail rings:
roll your own out of poster board,
keep an eye open for suitable product packing/packaging materials,
empty out a plastic jar of Skippy peanut butter (or some similar product) and cut a plastic cylinder from the empty container,
find a suitable hard smooth object to use as a mold and bend a thin sheet of balsa around it (laminate inside and outside with paper for reinforcement)