That front hub should have 9mm endcaps already.

A lot of aluminum endcaps do seem slightly fatter than threaded axle ends in my experience.

If it's just a tight squeeze, which I've found on some Rolfs I have, I just force em in there.

Another issue I've encountered on used frames is that sometimes the dropouts get squished closed a bit with rough handling, and will be tight at the opening, but a looser fit when pulled back into the dropouts.