I showed the image Terje made in this thread to show that if you have some metal-work workshop, it should not be too hard to build such a mount. You would need a 8 mm aluminium plate for the back, a 43 mm “Euro” motor mount like this one for attachment to a mounting plate, and two linear bearings that are easily available. This could then replace the spindle mount on the Onefinity Z assemby.
The problem one would need to solve is not simply to have a 43 mm diameter clamping ring, but to enable the Z slider to protrude downward beyond the lower end of the Z assembly, as shown here, what the Onefinity spindle mount is absolutely unable to do.