I have my 5c collet chuck now mounted into a cast iron Myford blank plate. The register area is about 0.1mm diameter clearance to the chuck body.
Made from some long M5 screws, recut the plain area to M5x0.5 thread. I used M5, then I could use a pair of 4mm keys to adjust the runout as needed.
The 4 clamps are made from the shank of a damaged M12 cap head screw. There is a step on the side of the backplate of about 0.2mm , so that the chuck body is always in contact with the small clamps. Their total movement is only 0.1mm, just enough to get the cheap collet to work.
So far am happy with it's functionality. I was going to leave the backplate at it's full diameter of 135mm , but cut it down, and the small clamps allow easy use of C snapper or strap wrench to remove from the spindle.
Neil