From your image, the ECX head looks more akin to a Pozidriv head than to a Phillips head. Does a Pozidriv screwdriver fit it?
As I recall, Phillips were introduced in the US for automated manufacturing - the driver was supposed to self align. In reality, the flanks of the driver are slightly...
Sand will work. Add a bit of glue if you want it to stay in place.
Glass fibre resin - be careful - it gets hot when setting.
Molten lead - don't breathe any fumes as they are extremely toxic. Don't use if it will damage the skull.
Bore a hole in the base...
Keep the chair stationary and shake the camera.
You will not hurt anyone and, as the camera is so much lighter than the person and chair, the device will be much simpler.
If necessary, use a "blue-screen" background and electronically (live or image processing) replace it with a matching...