Z-knob stop for MakerGear M2

This is another version of a lock for the MakerGear M2 Z-axis knob, to keep the Z-stage from dropping when the motors are turned off. The problem I've had with other such locks is that I have a tendency to turn on my printer and then hit the "home Z" button without bothering to check if the lock is on or not. If it is, then the Z motor starts hammering away, and usually I have to kill the power and start over. This version is designed so that it stops the Z stage from dropping, but won't stop it from lifting.

NOTE: For the M2 dual upgrade, this knob does not allow the extruder assembly to move all the way to the left. See http://www.thingiverse.com/thing:934680 for another version that solves the extruder assembly clearance problem.