Fabricate.. machine a solid aluminum plug just the right diameter ,
and thread the center in the 6 by 1 mm bolt standard, simple lathe job.
Epoxy it in place..
However, my cervelo 3t funda team carbon fork is built weird, in that it has an aluminum sleeve bonded to the inside of it. It has a 21 mm inner diameter.
the usual carbon steerer a star nut is Verboten ..
but as a workaround that aluminum sleeve lets you hammer one in ..
and so alternatively, simpler tools needed..
Take the star nut to your bench grinder
keep it cool,
quenching in water often to not remove the spring temper,
is a consideration .
take a little bit off the ends of the star edges..
[Math] Assuming they were made for a 22.2,, ID,
that's like 1.1mm off each end edge..
chuck a bolt, You screw into the star nut , into the drill-motor
cause it to spin in the opposite way from the grinding wheel rotation
and you are more likely to keep all the metal removed, equal...
...