Print both parts, use a large woodscrew or a tap and machine screw (I'm using extra M5 screws from my Mendelmax build, the hanger hole is 5.4mm dia. the mounting bracket hole is 4.2mm dia. and 15mm deep) to secure the parts together, use small woodscrews or bolts to secure the hanger to the underside of a shelf.

Magnets might also possibly work for those of you with metal shelves, or double sided tape for glass shelves.

I used a two-screw mount as my first attempt, but thought it was a tad flimsy so I made a three-screw mount and included that.

Feel free to play around with it yourselves.

EDIT: Thav seems to have found a potential issue with the hanger, so I increased the height of the catch nubs so things wouldn't fall off so easily and uploaded it as v3. His version is here: thingiverse.com/thing:29504 Thanks.