20x20 Test Stacker

I got tired of having a bunch of 20x20mm test cubes sitting around, so I thought I'd design a cube that:
- could connect to another cube
- test thread printing (M15x1)
- print with no overhangs (thus the cone in the middle)

Each one can be threaded on to another one. This also helps to determine if your different filaments print close enough to each other. Please post pics of your multi colored stacks! :)