Just occurred to me that you might be able to use a cast iron surface plate. Google the term and you
get lots of results. Will probably cost quite a bit more than a slab of mild steel but it should be pretty flat
right off the shelf. If you don't have to grind it the project will still be cheaper overall...
It isn't actually any BETTER than ribbed CI, but the 'go to' for a LOT of small chow and metrology fixturing and system "bench" mounts is granite.
For its easily shape-selected feature. Already has a billion years or so of stress relief "baked-in".... literally, and on the grandest of scale...as well. It CAN be broken, but still... worried about that?
Just go thicker. Strong enough to hold entire Continental land masses together ain't it? Even if hit with am atom bomb! Mind "thicker" is... kinda "relative?"
It will not NEED any further grinding at all. And yes, one can bolt stuff to it.
It now and then shows-up as salvage on ebay, even. With holes in it! Where the "whatever' had been mounted. But still, not ordinarily very MANY holes.
Grave monument makers can cut it to shape. Among the earliest users of water jets, they are. And wet-worked Diamond saws.
Herman surface plates CAME from a monument-maker. That was also "Rock of Ages" main line, historically... before granite surface plates were resurrected.
Ancient Egypt made them. Herman being "the first" is a joke by easily five thousand years!