I used 3/4" Baltic Birch (or Appleply - American equivalent) layered 2 pieces thick, for my table surface (5x8 machine) and 3/4" LDF for my spoil board. The Baltic Birch is very stable. We have below 0 winters and 90-100+ degree summers with varying humidity levels year around (almost no humidity in the winter and high humidity in spring and fall, with humidity all over the place in the summer). Nothing like the south though. LOL!

I always use a 1/8" piece of MDF as a sacrificial sheet on top of my spoilboard so I don't cut into my spoilboard. I cut a LOT (800-1000 sheets a year) of plywood and this setup works very well. I surface the spoilboard before each cutting job making sure everything is nice and level. I then surface both sides of the sacrificial boards to make sure they are aligned with the spoilboard surface. For smaller projects, I use a 1" piece of MDO with T-tracks. I can mount the project material to the MDO board then it has held down on top of the spoilboard with the vacuum.

I know you are not using a vacuum system for hold down but I had this system in place before I installed my vacuum system and it worked great.