Kamui Roku is my favorite chalk. However the Kamui chalk holder is eye wateringly expensive. I found a model for a belt clip that I can not find any more. Really liked the design, but the clip was too short and the magnet holes were too small. Can't find the original model now on the other site. This is my re-design of the same. Belt/pocket clip is 72mm. Made to use 18mm x 3mm neodymium magnets. You can find those at Home Depot.
I printed the clip in TPU and the chalk holder in PETG. Both in .2 quality mode with a .4mm nozzle You'll probably want to use a bit of super glue on at least the belt clip. Magnet is nice and tight in there - but the magnets are so powerful that they will work free eventually. Make sure to orient properly before final seating!
There's two models for the chalk holder. I started with a hex profile that matched the profile of the chalk, but I found that it was harder for me to keep a consistent wear on the chalk that way. Designed a rounded external surface. That's helped with making sure I wear the chalk evenly.
For me, the design is super tight, but that's intentional. Don't want the chalk falling off the clip. You might want to bump up the dimensions by 1 or 2% in the slicer.
The author hasn't provided the model origin yet.