I had a bunch of allen keys laying around form different toolsets, printer kits, etc. Now you can just store the remaining allen keys underneath whatever you use for single allen keys. It supports allen keys with up to 150mm in lenght and has a gridfinity-height of 3 (gridfinity-units)

The allen key holder stacked on top in the picture is this one: https://www.printables.com/de/model/676911-gridfinity-allen-key-holder