Minimalistic marker holder that should be fast to print. Open source, the code is here You can tweak some parameters, like how far they are spaced out, the diameter of the rod, how far the prongs go to hold it in. This only works with a certain branded marker. Not sure if I can Expose it for fear of getting sued.

EDIT: Made the post 9.25mm in diameter to make it loose-ish on a calibrated printer with correct tolerances. Changed the count of the nub things to 4 to match what's on the bottom of a marker, and made it a little larger.