Whiteboard eraser holder
Fits my Quartet whiteboard. The aluminum trim around my board is 12mm wide and 6.5mm thick.
This fits nicely to the side, but can easily slide up / down. A bit of tape or paper shim would ensure it stays in place.
This thing was made with Tinkercad. Edit it online https://tinkercad.com/things/dV5DIvKasDy