The holes are made on a mill using an end mill bit. That is how the holes are made round vs triangular in shape as you get when using a drill bit.
The single width roll bar holes are milled in a straight line just as with the full width bars, not in an arch. The 1-3/4" bars are done the same way.
We would prefer not to share the manner in which the lights are held in place or the way in which they are installed. We worked a long time perfecting this and prefer not to give away all of our secrets.