I've tried various methods of storing my Promarkers so that they'd be convenient for me to use. This method has worked best for me.
The green container I use is a case manufactured to hold 50 rounds of .50 caliber rifle ammunition. I'd looked everywhere for something like this and finally found what I needed at a gun shop! They cost around $5 per box; compare that to over $70 for the racks I've seen online!
The openings are just the right size for the pointed tip on the Promarker lids to fit inside and stand upright.