I designed these holders to fit 40-taper tool holders for CNC mills. 
They are modular and can be connected together to create larger storage structures.
The current version, V2.1, is tall enough to leave even the longest pull studs attached while still fitting all standard 40-taper tool holders.
The reason I originally made this system was because V1 was designed to fit perfectly into an existing slot on one of our machines. 
From there, it evolved into V2, which added dovetails so multiple rows could be connected together. 
V2.1 is slightly taller to accommodate the longest pull studs available.
Personally, whenever I have odd amounts of old filament left over, I use it to print these holders for the workshop.
I also designed a separate tap holder system, which is made to hold tap holders for tapping arms.