We use GoPros a lot during flight testing, and we get the best results when the camera is mounted as rigidly as possible to the aircraft.
If you're trying to video the cockpit as a whole, then the broom closet is a good place to mount.
If you're trying to video specific instruments, then mount as close to the instrument as possible. We often reinforce the glareshield and then mount to that. To get the zoom factor correct we replace the stock GoPro lens with one with a narrower field of view.