Loren, here's a wacky idea for you. You could use a regular ring
mandrel, and put the ring over the tip, but before doing that, slip
some (about 4) long, thin, narrow metal pieces around the mandrel, so
that they protrude past the edge of the mandrel, forming an extension.
Then, slip the ring over these, leaving the metal strips as a buffer
between the ring and the mandrel. Then you could hold the ring firm
and gently tap the metal edges on the ends (extended part of the
mandrel), pushing them further onto the mandrel, and taking the small
ring with them, thereby stretching it out...
Not sure if that makes any sense at all, my textual communication skills may
not be up to snuff. I'd be happy to email you a drawing if you like.
Port Hueneme, CA... Where it seems we are always under a marine layer!