Summary

The SmartALU has a poorly supported build plate, this is a relatively easy fix.

https://youtu.be/fHlEnPQbIsw

Requirements.
1x Lead Screw, 8mm Diameter, 8mm travel per revolution.
1x Nema17 Stepper, 1.8 degree 0.45Nm or more.
Some m3 bolts, m4 bolt and T-nuts.

Install:
You want to drill a 10mm hole approximately 310mm from the edge of the current z-nut ( 315mm) from the centre point. This should be outside your build area!

Drill two additional holes to matching the mounting specification of the Z-nut your have.

Mount the z-nut to the build plate.

Mount the Stepper loosely onto the Bracket, mount the coupler onto the stepper.

Install the bracket with stepper loosely so it can adjusted along the 2020. Keeping the stepper screws loose also so that fine adjustments can be made.

Lower the build plate till it touches the Coupler, insert the lead-screw from above, positioning the stepper + bracket correctly. Tighten the screws.

Connect Stepper in parallel with the rear z-stepper.