But then the 3x XPG2 may ultimately give you more lumen if driven to maximum ( 1.5 amp driver at about 10-11volt out put. In fact if you run it off a 12v NiMh pack you could use a AMC7135 based driver.... or go one step simpler and just use a resistor.
The head will be powered with 2x 18650 batteries (in series)
Any recommendation regarding the Led to be used, so she can get approximately 70-80 minutes burntime?
I have that optic, the transparent part is round and has a diameter of 32mm, the black holder has 6 sides and when measuring between 2 opposing angles, you get about 35mm, so I'm guessing that 35.8mm is from the holder.