As long as you are working on the right size hole, 43mm, the the shape doesn't matter as it has to deform only very slightly to grip the Kress.

My ' design' is based on a piece of 20mm thick 100mm wide alloy split thru into the hole at the front and a clamp screw fitted.
Even though this is very substantial it still grips the Kress hard, if it was 1mm oversize then I don't think it would deform enough to hold it securely hence getting it right to start with.

My design will not suit many as it's purposely over length as it has to reach on the OEM machine.
It also doesn't bolt onto the Z axis but fits on a quick release mounting so the original machine has a choice of three spindle alternatives.