I want to build a cheap mockup of my cockpit to check out seating angle, instrument location, control location, etc.

Just wondering what you guys are using to build a mockup. I plan on building the finished plane with steel tube, I was going to build the mockup with cheap wood but the only straight wood I can find in sizes similar to what the tube will be are trim grade wood, and that costs almost as much as 4130. Most of what I am seeing that is cheap in the sizes I would use are filler & spacer stock which is so warped it is only suitable to start a fire with. I thought about using 3/8" plywood but it has gotten pretty pricey & none of it is straight either.