Good suggestion.
Actually, I was trying to use the Aeria cap.
But reading your comment, I guess the issue is me just being too picky and expecting profile design to provide accessories for a perfect engineering fit ; )
As per your suggestion, I will use the M4 screw from Canyon for the time being before I machine a custom one .

Aeria cap with M4 screw.





I am still concerned about water being able to find its way into the headset tube though.(see blue arrow on the pic). I live in tropical climate. Combined with both the proximity of sea and the high mineral/salt content of local well water used for rinsing bikes , this makes it a very hard on equipment and favors corrosion.

I don't feel like removing the Aeria cap every time it rains or I wash my bike to dry the inside of the headset tube. Guess I will have to 3D print a small plastic cap to close the top of the steerer tube (blue arrow on the picture).
I also make a custom M6 head/M4 thread screw. Very traumatizing for me to use an M4 head in a M6 countersunk hole ; )) In our workshop, we avoid to do this in the outdoor equipment we manufacture as using undersized screw in countersunk hole creates a little puddle/space for water to sit on top of the head of the screw and exacerbate the corrosion issue.



Seems some suppliers on Aliexpress are actually providing Aeria kit with "head expender" (not sure how to call this) that would address the issue of closing the steerer tube. Did not see such option available on the Profile design website.