How to make an high-quality, planar waste-board

When I built my first Maslow CNC frame, I didn’t have much carpentry experience, and I didn’t realize how warped 2x4s and lumber-yard plywood could be. My 3/4 AC plywood waste-board turned out pretty warped. This is good enough for many projects, but now that I have an improved Z-axis stage, I want to try building giant printed circuit boards (PCBs) by carving 1/8 inch (3.2 mm) copper-clad laminate. [yes, yes, I know breathing fiberglass dust is hazardous!]

What are the best methods and available (and affordable) materials for making a highly planar waste-board/frame? Is MDF a good candidate? Are 2x4s hopeless?

torsion boxes are supposed to be very flat
many youtube videos

Can we see pictures if you succeed?

MDF is pretty good as long as there’s not a lot of humidity in your area.

Have you checked out any of the frames people have built using unistrut?