3-wheeled Y guide goes linear

This thing is superseded by my Y-Carriage_for_20x20_X-Rail_Version-2
by printingSome July 24, 2021, https://www.thingiverse.com/thing:4915787,
which is much simpler and, I hope, better.

I wanted to try linear rails, whilst enjoying NickRimmer's design, i.e. without much fuss and grand new designs.
If you have a 20x20 V-Slot rail for the X-Carriage, you can use this too with, for instance, my "Y-Carriage_for_20x20_X-Rail", www.thingiverse.com/thing:3401126.

I used 2 MGN9 350mm rails wit