Hi @wmsr, as you know from my thread, I’m looking for essentially the same thing, just a little taller.
The only ones I’ve come across so far in the 2" to 4" range are:
TEMPUR-Flat Foundation (offered in a 2" version). This is solid rather than slatted, but solid should be OK based on my conversation with WinkBed. I don’t know what’s actually inside the foundation, but it should be sturdy if it can hold a Tempur-Pedic!
Big Fig Ultra Low Profile Foundation (2"). Made of European Spruce. Again, the top surface is solid. From the photos, it looks like each half of the Split King has some kind of center hinge, but it’s hard to tell exactly.
I haven’t looked closely at these two, but they might fit your requirements. It seems there isn’t much out there in the 4" and under range.