My final optics and nozzle are sort of a hybrid. I use a combination of the LO 18~20mm Focus lens Mount <http://www.lightobject.com/1820mm-Focus-lens-Mount-P204.aspx? to hold the mirror and 18~20mm Laser head w/ air assist <http://www.lightobject.com/1820mm-Laser-head-w-air-assisted-Ideal-for-K40-machine-P701.aspx> to give me the lens mount and air assist nozzle. This part by itself won't fit properly in the plate, hence the mash up.
I was never happy with the fact that the nozzle was so far away from the work piece with the 50.8mm lens, not to mention potentially loosing power because the beam was still wide before it go to the waist. So I made on the 3D printer an extension to it. It's available here if anyone wants it:
https://github.com/sshwarts/2.xLaser/bl ... ension.stlPrinted in ABS which should be ok unless my alignment is way off. If someday I have a metal lathe, I'll turn one (anyone want to do one?)