The hammering is to make room for the unit.  The purpose of the bolt is to compress the bearing on the headset.    

I would not use the unit for compression.    
I suggest:

 1. Remove unit 
 2. Loosen pinch bolts
 3. Use the bolt (shown in video) to put proper compression on headset
    bearings 
 4. Tighten pinch bolts 
 5. Re-install GPS unit

Mainly I suggest you start with a seat tube based unit.  

Headset is a lot of fabrication.  In a headset during test you could just let the unit rattle around (pack it with some something).