*UPDATED: 5/3/2015 Made it taller to add more space between the magnet and the wrench. TOO STRONG :-). Tapered the top.
I designed this to hold an optical switch to be used as Z-Probe for a Kossel mini. I used a strong magnet that I had laying around from a hard drive I disassembled long time ago. The allen wrench is modified too, just a little shorter on the front so it doesn't interfere with the fitting. I got this optical switches on ebay:
The cones can be removed using small pliers or wire cutters and will open the holes for the fasteners.
Printed using repetier host and repetier firmware. Sliced using CuraEngine 0.2 layer and 20% infill. No support or raft. Borosilicate glass bed and Elmer's washable glue.