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

Hardware Frame
1

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?

2

torsion boxes are supposed to be very flat
https://www.youtube.com/results?search_query=torsion+box+
many youtube videos

3

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?