Should have made this customizable, but I didn't want to spend a lot of time on this, just a quick one off that my wife wanted for her eyeliners.
The openings are about 12mm square (snug fit for a Sharpie fine point).
The 'arches' are just there to make it look less boring. They are upside down because believe it or not, it actually is easier to print this way. When doing an overhang (especially the dramatic overhangs at the top of an arch), prints will tend to curl up unless you are using very heavy fan and keeping temps quite low. This curl up at four points on a very thin column can very easily break a column and ruin the print. The 12 mm bridges are by comparison very easy for a printer to do.