Gridfinity 1x1x1 Expo Marker Holder (10mm Pen or Pencil)

This fits 9 Expo Wet Erase markers (~10mm diameter) even tho they come in an 8 pack :)

Probably fits lots of other slightly-smaller-than-a-Sharpie 10mm diameter pens / pencils /etc.

Many thanks to user brian.kerley for his excellent Gridfinity Hole Customizer