Over the last few evenings,
A layer of mud follows to prevent sand from sifting down, then said sand (4 more inches) and finally red brick set into the sand.
We had a bit of trouble finding the brick. Many places we looked carried red-dyed concrete, but your basic clay brick was a bit tricky to find, and locating firebrick had us completely stumped.
Given the intermittent use we hope to get out of this oven, we think the red brick should work just fine. It looks great…and we can’t wait to cover it up!