two sheets of 25mm MDF, a 4x1 frame between to make a torsion box.
Paint with iso glass/colloidal silica to fire proof.
or buy a cheap 3'x2' cast iron surface plate.
or...
dunno how well this would work? a granite headstone. (preferably blank/unused - not off a travelling tarmacker)
It’ll be flat enough for those easily.
Any flatter and your in surface plate / table territory.
Here are “some” of mine:
View attachment 160546 View attachment 160547 View attachment 160548 View attachment 160549
If your sheet of 10mm is reasonably flat then use that to lay out a frame of 50mm shs.
Tack the entire frame before welding it all up.
Once the frame is welded, stiff and square then tack the 100mm plate onto it.
Thanks again. Out of interested would you be interested in building the top plate for me. If so let me know how much £££ you would charge as it would be nice to just get it done without me messing it up again.