Update 2 june 2021. Read the Disclaimer.
On this page I have collected my experience with creating a Gopro Hero 9 mount for Fox Proframe full face MTB helmet.
My helmet is a size L !!! I don't know if it fits different sizes. I didn't like the upside-down camera mount attached to the visor of the helmet, although much simpeler to make. With this mount the camera is in its normal position, has better GPS reception and its possible to turn it on/off with a finger. The files are created with FreeCad. Its printed in PETG on a Creality CR-10 with 0.2 mm layer hight. To get nice curves of the curved sections its printed standing up-right on the heatbed. Printed at 257 degrees Celsius with the heatbed at 80 degrees Celsius.
The stl files can be downloaded here but are also on ThingsIverse.com https://www.thingiverse.com/thing:4875996. Gopro mount , mount simple Fox , mount simple raised Fox , click-on chin mount Fox . I have tried a design with a bayonet clamp but I did't like it because it was more difficult to free/unlock it. If you do not have a 3D printer then Google for a 3d print service.
Warning: 3D printed parts are never as strong as an 3rd party injection moulded part or official GoPro parts.

The finished camera mount.

The design is made with FreeCad. First the 2 curved parts were designed seperatly in itterations: design, 3D print, if needed adjusted curve/sphere, 3d print, etc. Once that is 'good enough' they are connected to flat 4-5 mm thick rectangle.

Heat the M5 cap nut before pulling it into the mount. No glue needed.

I used double sided mounting tape to fix the mount on the helmet. Its foam based and can be cut with a thin knife. Perfect for testing. Heat it with a hairdryer for maximum adhesion.

Secure the mount with 2 cable ties, just in case. And maybe also the Gopro to the helmet with a loop from fish thread...

After some thinking: why not a click-on mount ? No glueing, just click-on. Until now some 10 houres of riding with the mount not secured.
Check ThingIverse.com https://www.thingiverse.com/thing:4875996