Magnetic pool cue chalk holder that clips on to your belt.
Once saw someone use similar and thought that would be easy to create Makerbot style.
As for a magnets I had a 20x2mm disk type neodymium magnets at my disposal. Strong enough to clip together thru a shirt.
The chalk cube itself fits snuggly in the box.

*If enough interest I can (or anyone else) modify the design to use the 3mm cube magnets that makerbot sells, but since I don't have those at hand I can't verify if they'll work.

Print both parts
Clean the supporting structure off the box part
Insert magnets and chalk
Play kick ass game with cue chalk always at hand