This design is for a chalk holder. It is assumed that 21mm, 22.5mm or 23mm square chalk cubes are used, which are common sizes. The chalk is held within the holder by friction tabs located inside. A hole at the top is provided for pushing the chalk out or for using string (e.g. paracord) to attach the chalk holder to a wall, cue rack, etc.

The OpenSCAD file is provided for any needed adjustments.