I have a Longmill Mk2 and had a fixed length laser module from Sainsmart lying around so I thought I would add it. First look around and the mounting is different than what I have found in my searches. The holes to add on are on the front rather than the side. This complicates it because it needs to fit nicely between the two screws that hold the router in place. Second issue was that the Sainsmart laser uses 6 holes on the back, not 3. So I fired up my CAD and drew one out.
Sienci Mk2 Mount for Sainsmart Lsr.stl (35.6 KB)
The two files are the STL for 3d printing it, and the other is an SVG file to cut it from acrylic.
The setup for the laser is exactly the same as the other examples. My laser control board has a 3 pin connection (pwm, gnd, 12v) so I grabbed 12v from the fan plug on the longboard, and used the pwm from the spindle. Works great.