The carriers sit flat when the outer bearing opening is up so I chucked this carbide burr in the small benchtop drillpress.
Then I just adjusted the height so that the bit just misses that lip onto which the bearing sits and I slowly remove material going in a circular motion, doing my best to stay center. Not the most precise way of doing it but it works.
The inner bearing doesn't sit in the 18 mm space for the stock 12X18 bearing, the round opening in the carrier before the space for the stock bearing is just the right dimension to press that 15X21 bearing in. This bearing will sit on the lip create by the stock opening for the 12X18 bearing.