A simple six-marker holder. I wanted to make something that looked like it came with the board from the factory. Just stick it to your board with double-sided foam tape.
I found that this printed fastest when flat and at a 45° angle with "Lines" infill. Using 0.2mm layer height seemed good enough, but 0.12mm would be better if you don't mind a 12 hour print. The inner diameter of the holes is 20mm, so you may need to scale this up a little if you have larger markers. I tested with several different brands, including EXPO, and they all seemed to fit fine.