Hi Comp625,
I’m not sure which foundation TandL was linking to (it goes to a reply link to this topic) but there are some sources for KD (knock down) foundations in the foundation post here.
Just as some food for thought there is more about solid surface foundations in post #10 here.
If you put a solid sheet of plywood on a box spring it would provide good support but depending on the specifics of the mattress or the box spring it may still sway or tilt when you sleep on one side or the other or with heavier movements (particularly with heavier weights or weight differentials between a couple). As runnerb0y mentioned if you have a split in the middle under the heavier part of your body it could still deflect under your weight at the split. At the very least I would make sure that a split wasn’t under the heavier parts of your body (as runnerb0y also suggested).
Outside of ventilation … the goal is to have solid support under the mattress that is stable and doesn’t flex … particularly under the heavier parts of the body.
Phoenix