The holes are 21mm apart but I wonder if you drilled (milled) them in or out to 20mm or 22mm would you run in to any traces or electronics and kill the board?
There is a lens holder designed specifically for the raspi camera. It is machined out of aluminum, has the correct hole spacing, and has a cutout for the cable to get out. It is $8.50. I'm using two of them. This is the holder for the V1 raspi. They have one for the V2 also but it has no picture. I'm sure it is as well-made as the V1.
http://www.m12lenses.com/CNC-Machined-R ... 020rpm.htm
The site, m12lenses.com, has a great assortment of lenses and lens holders. I spend too much money there.
I have 6 different focal length lenses to play with.