#1. By using a blunt point allen key and poke gently through the all the holes in the side. Just be careful not to poke the sapphire window (nice thick chunky window as well.). Window would just drop as it is just seated on the inner o-ring. To install just do it reverse way, extra care when seating the window on the inner o-ring. As it is a tight fit, the o-ring expands to its ori size when removed. So seat the window and use thumb to press it squarely/firmly. Then install outer o-ring and use index finger to seat the o-ring inside its groove. You will now the o-ring is seated right when you see the beam pattern.
#2. I too have no idea. Tried to match one using a purple carbon graphite and found out that its too small to be manually constructed (ie using handrills etc). Only way for a nice perfect fit is by proper machining. Oh and the thickness of the intended disc around 1mm. 2mm and its above the emitter's housing. A mirror cut to size I think would be cool also.