I’m installing Planet Bike Speedez clip-on fenders to My Cyclocross bike. The fork is a carbon fiber unit with an aluminum steering tube. This fork is produced by Ridley. The fork does not have attachment points for fenders. There is nothing for attaching fenders at the lower section near the drop-out. There is also not the usual horizontal hole that passes through the crown. The fork has canti mounts.

There is a vertical hole that is under the steering tube. The hole is pictured below and is exactly the size of a presta valve cover. I’m considering using this as an attachment point for the fender. The fender has a series of holes along the top and one of these holes lines-up right under the steering tube. I could fill the hole and use a stainless sheet metal screw to attach the fender. This would stabilize the fender at this point and improve clearance by holding the fender against the arch of the fork crown.

Bad idea? What glue would be best? Should I use a valve cap like a plug and screw into that to secure the fender?

Speedez fenders attach with straps to the fork legs, nowhere else. I don't understand what you're trying to do.

The fenders have straps for the blades of the fork, no problem there. But the fender would be better aligned and silent if I could attach it to the crown.

I wouldn't plug that hole permanantly but you could use a plastic wall anchor or small Moly bolt to anchor the mounting bolt for the fender.

Or a rubber cork..
But,, you bought a race bike fork, then your needs changed,
why not just buy a replacement fork with the features you now desire?

Ridley is another jobbed out contract filled in China,
and they take over marketing and distribution
after it arrives in their warehouse. it's commonplace now, nothing new.

I actually purchased a Surly CC fork that I plan to use for touring next year. It's a boat anchor compared to the Ridley fork.

.. threadless forks are a quick swap, take maybe 20 minutes,
so pull the fork and the mudguards out as a unit when the race happens.

I was thinking of a plastic drywall anchor like hillrider suggested... cheap, no risk of damaging the fork if you aren't stupid, and will hold a fender in place.

Yes, I'm going to try this. I'll clean out the hole, use a little silicone to help hold the anchor in place, and let the screw expand the anchor a little. It should all be easy to remove if needed.

a Daruma is a part for fitting fenders via plugging the bottom of the steering tube,
almost .. as said in #4, a little drainage hole is good to let condensation drip out.

That's the idea, open link and scroll down: https://veloweb.ca/garage/how-to-install-honjo-fenders/