I made a marker case /pen holder for some Pebeo watercolor markers but I made the spacer removable so that it can be removed or adapted to other things. The top case has a hook that you can use to stabilize further the case in pen holder mode and expand the capacity.

I printed the case in Form Futura PLA but PETG might be better suited (because of its resilience) if you intend to carry the case a lot.