The GoPro can be easily mounted on the Speardiver Stealth Mask using the compatible GoPro Mount.
To mount the GoPro on other masks I use the Gopro tripod mount. Drill a hole in the mask for the tripod mount bolt, cut the bolt to correct length and that's it.
In the water you can't feel the camera on your head at all, the camera points exactly where you look, and moving the head looking from side to side does not create jerky footage. It's easy to reach for the buttons by feel, and the beep sound of the camera starting to record is clearly audible.
Another couple of advantages of mounting it this way is that you can easily pivot the housing down and away from the mask to be able to open the back of the housing and remove the camera from the housing. And that you can easily remove the housing/camera completely leaving just the tripod adapter on the mask, it doesn't weigh anything and you won't even remember it's there.
A speargun mounting system from FreediveStore.com - GoPro Speargun Mounting Bracket