Edit 5/12/2015: I updated "Z_Probe_Effector_Just_the_Spring_Mount_End" with a "repaired" version. Nothing has changed in the design.
Edit 12/4/2014: I have added 8mm versions, so that there can be a straight-across replacement for the current Cherry Pi III setup.
I Rebuilt the end effector for my Cherry Pi III to use the sprung-string approach.
I opted to change my 10mm magnets out for 10mm ball bearings (instead of the pre-drilled 8mm balls implemented originally), and made some other changes.
I can verify that this just fits an E3D Mark6 fan and clip below the platform. There are new holes for the springs on the J-head mount, as well as two corresponding holes on the platform.
I also moved the hinge for the rod carrier to the top of the carriage, giving back most of the z-height lost by moving to a top-mounted effector design. This is done by flipping the mini V-wheel mounts upside down. The holes then line up.
Also, note that the diameter of the balls, or many other measurements, can be edited within the OpenSCAD files to adjust them as necessary. So a version with 8mm balls can be used as well.
I used the cupped-out rod/screw combination from my original magnetic implementation, but replaced the magnets with 10mm ball bearings. I had had problems with the metal surface of the magnets de-laminating and then the magnets themselves beginning to disintegrate. The solid steel of the bearings is much harder and designed for this sort of stuff.
It's a little difficult to get the arms loaded with free-floating ball bearings, but I coated the ends of the rods with a fair amount of grease before pushing a ball on.The ball would form a slight vacuum stick to the rod, helping to hold things in place for assembly.