Better 3 axis calibration cube

This is a precision 20mm XYZ calibration cube which improves on the typical calcube. There are two issues with the typical cal cube:
1) Hard to make an accurate Z axis measurement because either you have error from the squish of the first layer or the uneven surface from the raft/support
2) Over/under exposure/extrusion can result in a growth of all dimensions by a small amount which is an offset not a scale error. When you measure two outside edges, this error is additive and is erroneously