I watched @SuperDuperCruizer do one and he used a non-corigated piece of cardboard (cearal box maybe?) and wrapped it tight around the ring like a piston ring compressor.
@SuperDuperCruizer did a really good write up on the rebuild process.
Found it....
https://forum.ih8mud.com/threads/40-series-power-steering-gearbox-ssts-rebuild.113223/