This time, I disassembled a springform pan. I crumpled up 4 alumnim foil balls and placed them in the bottom of the Dutch oven, then put the bottom of the springform on top of them, to create a raised surface inside the oven. The sides of the springform served as a charcoal ring for the Dutch oven lid.
The bottom did not burn! But it was underdone, again. And in an attempt to let it cook through, I left the bread sit in the oven - apparently too long, because it got really really tough.
Probably worth trying one more time, maybe with a few more coals, and a longer bake time.