I have 1,78m and the headrest of my IKEA Bästboll was too low for me, so I knew I neededed to develop a headrest raiser.
I found this model that was nearly what I was thinking, but it was excessively tall: https://makerworld.com/pt/models/2533345-ikea-bastboll-headrest-extension.

So I remixed it so it only raises 5,5 cm (max) instead of 9,3 cm.

This makes the model sturdier, less springy and to use less filament (and printing time).

After printing I've made a V2 to add a rounded cut on the model top so it doesn't show through the hole of the original chair headrest (not visible in the photos of the printed result, but visible in the model image).

Additionally I added the 3 screws and nuts to mount it, if you need them while you don't find proper metalic ones. I've been using the 3D printed ones with no issues!
Print the screws and nuts in diagonal orientation so they are more resistant. You'll need supports for them.
 

I printed mine with BambuLab's PETG Basic Black.