I wanted a caddy for my little selection of Posca/Gunpla paint markers, and needed it to be:

• Able to keep the markers lying flat(-ish) for storage* but can also be turned upright for easy use
• Adjustable for the number of slots, to suit my typical marker supply
• Compact as possible, to fit my cluttered workspace
• Simple, sturdy, and relatively fast to print

I couldn't find an existing design that ticks all those boxes, so I made one myself. I posted three example STLs using different parameters, but the OpenSCAD file has been formatted for the built-in Customizer, so feel free to tailor the design to your liking. 

The default values in the file + posted example designs use a marker length, diameter, and tilt angle that are optimized for a standard paint marker, but also work fine for regular pens/pencils as well. Likewise, the default wall thickness is 1.8 mm to print clean continuous-extrusion infills with a 0.6 mm nozzle; I'd recommend dialing the wall thickness back to 1.2 or 1.6 mm if you're working with a 0.4 mm nozzle.

The feet print at the top as an overhang in the default configuration, but setting a steep tilt angle causes this overhang to get pretty extreme. If you want an angle >20 degrees, I recommend printing it with a flush top, then flipping it 180 degrees so the feet print at the bottom instead.

* N.B. It's debatable whether paint markers are significantly affected by being stored flat vs vertical, if you have an opinion let me know in the comments