I found a *very* thin piece of foam, it's probably the same thickness as a paper towel folded once. It's the type of foam you find some electronics wrapped in. My PCB felt "wavy" even without the foam. I'm not sure if my PCB was a bit convex or the struts of the case a bit concave, but the foam filled the gap very nicely.
I also made extra large holes in the foam around the screws, over 1" in diameter, which allows the PCB to be in closer contact with the case at the mount points.
I'm very happy with the result. There still is a bit of a flex when I hold a key down hard, but unless I'm staring at the keyboard I don't feel it and most importantly I don't hear it.
It was the noise that was driving me crazy... without the foam, the PCB made a slight noise while bumping against the case struts even while typing on cherry reds without bottoming them out. The flex was almost imperceptible to the eye or hand, but you could hear the impact of the parts coming into contact!