So my original plan was to build a wood base/frame for the platform (engineered sturdily enough to handle the weight of the cob and the water on top) but I started getting nervous about hot spots and burning the wood. It would be easier if I could build a smaller, thinner cob platform on a wood table. The original plan was OSB bottom and sides, layer of foam insulation, 3" of sand, 6" sheet metal ducts on top of the sand and sand surrounding everything. I planned on running a few tests to check the temperature of the top of the sand, and as needed, I could add cement board to distribute the heat or a layer of bricks for more mass and spacing if needed.
I found out/heard that sand won't work well as a heat storage material, too much insulation and I'll be likely to still have lots of heat exiting my duct even after close to 30'. A history buddy of mine who uses cinder blocks for everything was thinking of the roman baths and suggested using cinder blocks as both the fill material and the duct material (you'd line up the cinder blocks so the gaps made a channel and the exhaust gases would run in those channels (with some custom made something that did the u-turns at the end of a row of blocks). But it sounds like cinder blocks and most cement stuff can't quite handle that much heat.
So, I'm back to cob, but would really love to not have to come up with a yard of cob. If I'm just doing an inch or two of cob under the ducting, as much rock/urbanite fill as I can find along with cob to fill in cracks, around the ducting and then 4-6" on top, that should be fine? (i.e. no wood burning and no liner melting

That comes out roughly to half a yard of cob (assuming I can add in rock/urbanite as available).
Anything else that would work as an easy to make/find material for filling in around the ducts, catching the heat from the ducts and holding it/passing it to whatever rocks/urbanite I have in there?
Thanks for any thoughts