I have a LOT of markers and need to keep them at close hand. There are four rack flavors that can be grouped, including flat, tiered, offset and aligned. The aligned have adapters built in for ganging them together. The offset gang together naturally.
I've included a single module in case you want to create your own configuration. All of the groups can be printed at less than 100mm x 100mm, for small printer peeps.
There is a bit of wiggle room in the slot so the caps don't get lodged and stay in the slot. Additionally, the bottoms have an offset, to keep them level, but are open in case a marker goes bad... which they often do! This also allows you to wall mount them with standard screws.