I'm just on this forum to ask a question myself. but my eye fell on this question.
as a goldsmith i'm well known with two mechanism designed to stretch the rings (like weddingbands) into a larger size without having to cut the seemsless bands.
a similar (or these instruments themselves could verywell fit your needs)
one (the cheapest once) is this design.
a long metal hollow conus is cut into 4 long strips and expanded by pushing a similar conus from within, intheory leaving not an exact round ring (in practise it does thought)
the other design i have no link of but is also a steel conus. but with strong steel prongs from the small diameter pusing the ring down to the larger diameter, stretching the material this way.
sorry if this was useless info but, ifso just ignore it.