Mike Jay wrote:
So for the block arch in your first photo, once you get a couple feet into the interior of the arch the floor should come up (sand/rubble) and end up an inch or so below the bottom of the pans. That will match the chimney cross section and force the hot exhaust gasses and smoke up against the pans to give a better boil.
The steam pans are fairly affordable on-line ~$25 each. Or cheaper from restaurant supply places or the dump.