This model is designed to adapt a generic headrest available from various online retailers and install it on a Haworth Fern. 

The adapter does not require any additional modification of the chair and can be added and removed in a few minutes. 

The system was designed to use the included clamping screws to mount the headrest to the adapter, then M5 screws to mount the plate to the Fern compatible mount. M5 x 10 is suitable for the adapter plate to mount, then M5 x 20 (longer can also work) to clamp the bracket from underneath. There is space in the model to use nut inserts for the adapter plate, I found that screwing in to the print just didn't offer the confidence I wanted in the headrest under load. 

You will also need some sort of foam or felt where the lower part of the mount attaches to prevent scratches on the chair and add the needed pressure to mount securely. 

I printed this in PETG for the upper, but am still using the lower part of the mount printed in PLA. It may be possible to print the whole adapter in PLA or PLA+, but I did not try this myself. 

I printed on a course layer height, but you should be able to print at any resolution. 

The model does require support around the cut-out for the headrest height adjustment pin.

I've included both STL and STEP files, I'm more than happy for the community to remix as needed! 

The headrest used was purchased on Amazon, but when I was looking around for one there were a bunch with the exact same design under different names. Here is a link to the exact one I purchased https://www.amazon.com.au/gp/product/B09KXQ9355