The plans are not available, (and @Luke can probably confirm whether they would ever be, this is not an open hardware item as far as I am aware). For what it’s worth, I did a (very incomplete) Fusion360 sketch of the back of the HDZ based on my own measurements and posted the design file in this thread.
I was only interested in two of the tapped holes near the top, and the location of the two holes at the bottom that hold the bottom plate. My frame thingy designed to these dimensions was assembled with no issues, but the dimensions may still be slightly off, since I used generous tolerances on counterbores, and parts made of (3D-printed) plastics to not have to worry about that.