Ok so far:
4.25 overall height
4.5 overall height
6.0 overall height
I was going to use mdf but Lowes mdf was very ratty. I picked up two sheets of 3/4 maple ply. When I got them home one is straight as an arrow the other is warped. I will return the warped one and get another that is straight. The warped one would most likely work but fot $66.00 a sheet Lowes can replace it. It will be a few days before I can return there is a winter polar vortex for the next couple of days so I dont want to get it wet.
I think I will make the grid 3.5" with 1.5" of skin making 5 inch height. Since this is not a work bench I will not cover with tempered hardboard. I will put down 3/4 inch ridgid foam under Shapeoko to support base. Will remove leveling feet.