Steel plate for hexapod bed

I'm trying to install a decent flat, stiff bed on my hexapod.

I bought a 24" x 24" x 0.25" piece of steel plate from Metal
Supermarkets, but it's not flat. In fact, it curves away about 1/8" at
the edges. Is this expected from ordinary steel sheet, or should I
demand my money back?

How can I get a decent flat bed without spending a fortune for a ground
surface plate or spending the rest of my life scraping?

Simon