When I printed the MMU2S Buffer (inspired by RMU Buffer (well actually I printed this Hex remix) I found the 608 bearings were super loose on the axle and, as such, the buffer wheels could move around on the axle and come in contact with the sides of the buffer box.
I drew up these shims which not only hold the inner bearing race to the axle, but also hold the buffer wheel centered in the space.
You'll want to print them slow and fine as the sidewalls a pretty thin.
Unfortunately I don't have any specific images of these shims as I've re-homed the buffer I built.
The author marked this model as their own original creation.