Ever consider pearite?
We oven builders use it with portland to make insulating concrete.
It is very light and absorbs very little moisture (also used in potting soil for added "volume" and helps hold air)
Just my .02...
You would of course have to line the pearlite with something to hold it in place or protect it.
Back to the discussion of the "double wall".
42" WFO,-Brick vert smoker, Tuscan wood grill, 22" Weber, ECB-sent to the curb! Superfast PINK Thermapen
Offically a ZERO
"When it comes right down to it, you just want to be surrounded by good people and good food."