I’m doing this for my system and I think it will help in the overall assembly of a new Shapeoko 3 by a new owner.
I am installing a new base plate (the mdf board that came with the Shapeoko 3). It is a 1/2" thick sheet of PVC from home depo. I am recessing this along the front and back edge 1/8" down for the width of the front and back rails so that the mdf waste board will sit flatly on the pvc board for the entire surface of the base board. This will result in complete support of the underside of the waste board and since most people have their system on a bench, table, torsion box or what ever flat surface than the flex that is inheirently included in the system will be eliminated.
Recessing the front and back edge of the base board will cause the front and back rails to line up square along the recessed edge of the base board. Starting with a square base, it should be easier and less problematic assembling the Shapeoko3. For those who want to use the leveling feet,one could install this base board upside down or maybe the recessed base board should be intended to be on top of the rails.
Maybe this is entirely unneccassary and maybe it will help, …Jude