Stacking calibration cube (SCaliCube?)

After a while of printing, experimenting and trying out different filament brands calibration cubes start to infest my work place and beyond. So I tried to make cubes that can interconnect with each other.
This is my attempt to:
* prevent cubes to scatter around
* avoid "Why you keep printing useless cubes?" question
* make calibration cube's afterlife more "fun"

Cube edge length is 20mm. Square hole edge length 7mm.
Included:
* simple cube with single pin
* separate pin for broken or failed cubes
* pinless cube just in case

(It's not perfect. Maybe I'll improve it some day.)