I designed a new adjustable endstop flag for optical z-endstops. I use two of them on each side, to level the bed and synchronize the two z-motors.
The knob and the mount for the optical endstop is taken from "tessarin": http://www.thingiverse.com/thing:3272343
Print it in the orientation seen on the picture. You will need a small support on the fly housing.
You have to mirror the flag housing for the left side.
To assemble you need:
2x self locking nuts 3mm
2x hex-screws ca. 35mm
2x strong springs (pen springs will do in emergency)
If the flag moves too strong in the housing, grind the walls just a little bit.
If you paint (Edding) or print the flags in black, they trigger better.