Hey Allan,

We used neodymium magnets (I'm not sure of the exact size and the guy who designed that is out on vacation currently), but they're designed to be press-fit into the slots from inside the case.  If you scan this ThingMark - it will give you an Augmented reality view of exactly how to build a Sigma Tile.  The ThingMark is designed to be mounted on the top of a completed case, but it will work no matter where you put it.

Hopefully this helps!

Milan