I'm sure there are folks who have info like this already but I wanted to share something I found.
lantech.com has sleeves that are 12mm od 10mm id and 10mm long. p/n 000457A
I based that on an 11mm thick flange for mounting the ring gear.
If the flange is thinner than that, p/n 000456A is the same as above but 8mm long.