I ended up watching some videos of people with a MPCNC on youtube and went with the really nice MDF 3/4. I still used a sheet of OSB 7/16? on top of the table then the MDF on top of that. I might use the extra MDF to make a “work piece/ spoil board” but what ever I end up doing I want to face it because no matter how hard I tried the surface still deviates some.
I didn’t use the top to square the frame,I put it on my concrete garage floor, wrapped a ratchet strap around it and pulled it tight. Once I got it square I screwed it tight and removed the strap then screw the 2 tops down.