This is all buttoned up with a hole in the belt cover for the bracket. I now have 6 knurled point set screws with Loctite holding the supporting ring in place. I also swapped out all the set screws in the z-axis drive with these. Repeatability is good except for when the quill itself rotates it varies about 0.005". The screw-pin that prevents the rotation has some play, so making a new one would help, and maybe a spring.

Edit: new screw turned down from a cap-head with a nice snug top and half a turn of tension on the spring and it's 0 to 1.5 thou variance from quill rotation along the travel, and the backlash is gone.

Also the quill return spring is needs more tension - the drive needs to push it up at the top of travel.

The scale mounting is more rigid than it looks - there is a second screw holding it to the bracket at right angles to the visible one. The head on it is low enough to let the visible screw pass over it.