This is where decent hand skill and a well suited file are your best friends.

If you don't have one, buy a RAT-TAIL file of medium cut (too fine and you'll be there all day, too coarse, and it's easy to over file), and a diameter as close to, but smaller than 9mm. Now it's only a question of removing a bit from the tip of the slot in the longer blade, so that side of the axle goes deeper, moving the top away from that blade. Note that there's something like a 6 or 7 to one ratio for rim position vs. slot depth, so file only a bit at a time, and test often. Finish by nicely shaping the oval, keeping in mind that you want a U not a V shape so the axle is solidly on then bottom instead of cradled. If filing directly into carbon, be sure to seal the exposed edge with lacquer.

Of course, if you do go a bit too far, you can correct by filing from the other side. However don't make this a sketch comedy routine whereby you try to eliminate rocking of the dining table and end up with a coffee table.

BTW- if you have bad hand-eye coordination you can improvise a guide to keep the file square, using a 5/16" OD tube forced over the tapered end of the file,


FWIW- Online offerings often fail to provide details of diameter or cut (they give length which you care about least) so I suggest going to a store where you can see before buying.