1950's table saw as DIY CNC bed...

[Toolmaker51]

There's always Dave Gingery's method for handscraping a metal bed into near-perfect flatness.

Ol' Dave G. only proliferated it to a audience beyond 'us' in the know. The 3 plate process, it goes back centuries.
The most visible examples are probably Whitworth's metal planers, mid-1700's. He'd attained flatness far below thousandths, before any instruments that could measure such deviation.
Very same phenomena brought back English Wheel, demonstrated frequently by Indian Larry, when reality TV was truly inventive and informative, not ridiculous drama.
Seeing the new posts collect in this thread, cast iron is a natural choice, aluminum not far behind.

My choice, starting from scratch would be different. I'd build on one version or another "acrylic polymer and alumina trihydrate", quartz/ resin, or marble countertop materials. All are very flat, virtually impermeable, some quite machinable and will hold tapped holes, or inserts. Visit your local building material upcycling depot, new material not required.
That atop a suitable ballasted frame isn't far removed how many modern machine tools are made - linear guides on simple cast beds.