parametric mendel z-axis top holder

This is a redesign of thing 4101.

For a long time I was wondering why my z axis is wobbling although I centered it perfectly.
The answer was quite simple: My printing speed is set too high.
Due to this the whole mendel is shaking very much when it is printing small parts.
This causes a little shift in the z axis, which is only fixed at the bottom.

The result is that layers do not fit perfectly in z direction. It is only about 1/20 mm but one can see it as relief on prints with a layer t