OpenSCAD rhombic dodecahedron tile with holes for magnets.
You'll need to print a total of 12 tiles, and 4 disc magnets per tile (axially magnetized). That's 48 magnets in total. I bought mine on eBay in bulk and used superglue to secure them in the tiles.
See the User Defined Variables section of the OpenSCAD script for the parameters you may want to change (magnet holes and overall size of tile, total 4 parameters). Or just use the pre-built STL file (holes for magnets are 6.5mm diameter and 2.5mm depth, designed to accommodate 6mm diameter magnets with 2mm depth).
Not familiar with OpenSCAD? It is easy to obtain and use! Download it here: http://www.openscad.org. After installation, open up the OpenSCAD file for this thing and hit F6 to render (or use the menu options up top). File->Export as STL and take it to your slicer!
These are also on github: https://github.com/eshira/polyhedra