This is a pretty basic remix of the HD brackets I had already printed and the inverted Z dual screw setup I wanted to install, which wouldn't fit together. I cut out a space for the stepper and replaced some re-enforcement so it should still be pretty stabilising. I left the bottom open to facilitate installation but my machine has slightly larger steppers than the model I was using to design it, so they turned out to JUST fit, and the normal steppers will have plenty of room (see picture of first version with a normal shorter stepper.
I created a version that has custom supports for the motor bracket that were designed for a 0.5mm nozzle, should work fine on smaller nozzles too. It allowed my printer to print the overhang, but not necessarily any better than with slicer generated supports because of an unintentional big gap between the support and the bracket - now deleted.
This is my first adventure away from OpenSCAD - right into MS 3D Builder... meh... mouse inaccuracy.
Next morning: decided to add a bit more strength back in to the design... probably would have worked just fine without it but since these brackets were meant to be super stable, might as well keep it that way as much as possible. I also re-did the supports, so they provide actual support...
It is now installed and the only other thing I had to do was to reverse the stepper motor coil wires so that it runs in the opposite direction, otherwise you need to re-install firmware to make the change in software if you don't want to make a physical change.
I was getting bad z banding / ribbing, tried many things to fix it, but currently replacing the springy coupler with a new one and making sure the rod and motor are actually perfectly aligned seems to totally fix it. I will be trying another oldham coupler design as well that should work a bit better than the extended nut mount, and will be more necessary once the rest of the hardware for the dual z setup is installed.
I also needed a taller z limit switch mount because the needed screw was far too long - but probably also exasperated by the fact that I added a linear rail to the y axis, so I also added a shorter one than I needed which may be more suitable for your setup.
Edit: May22/2019 ... I have re-created this, basically so the motor mount is more able to hold it further away from the printer so it might (untested) work with an normal oldham coupler that isn't quite as tucked up against the x carriage. I uploaded the Microsoft 3D Builder .3mf file that contains the chopped up motor mount so if it's not quite right you can adjust it.
0.3mm layers x 0.5mm nozzle
MG Chemicals PETG
No supports needed if printing the version with added supports, but you'll need supports for the normal version.