Years ago, a company use to produce a flexible rubber mandrel.
Unfortunately, they would wear down over time and then the finger
sizes would be off. People complained and the company decided not to
make them anymore. Since then I’ve run into this several times and
have found avery simple fix.
Take your solid mandrel with good markings stamped into it (numbers
and lines for the different sizes going down the mandrel). Next,
take a sheet of plain printer paper and roll it around the mandrel,
make a mark on the paper where the paper overlaps about 1/4". Cut
the excess paper off and roll the paper back around the mandrel and
tape the paper together. With the paper still on the mandrel, take a
pencil and rub it up and down where the markings are stamped on the
mandrel. Voila! You now have a flexible mandrel.