Drilling plexi is a challenge. One way is to make sure you have an appropriate high speed drill with a very sharp bit. I usually tape over the spot that I plan on drilling too so that there are no shear edges. If you are not overly fixed on screwing it onto the panel, glue does a great job.

I more expensive but reliable option is to have someone like FPE make the windows for you too - they can cut any material you want and even etch onto plexi to - if you underlight the plexi window, whatever design is etched onto the window will light up

..dB

Transplanted drivers from TDS-5M(russian HP-1 clone) into the PMB-80 enclosure. Don't worry if you have no clue what either of them is, these aren't exactly garden variety headphones. 
TDS-5M drivers were chosen because they are of same size as pmb drivers and are reputed to have better bass. They also don't need much damping to sound good.




This is a 55mm ortho driver essentially 'hanging in air' to allow mixing of front & back waves. Micropore tape is applied on the 360 degree vent to ensure that the mating isn't too destructive for bass.PU foam ring is used to secure the driver on the baffle struts to prevent resonances.

The result is a headphone which has the best soundstage I have ever come across, very wide & deep & immersively holographic.People who believe that orthos can't do soundstage & air are in for a big surprise!