This method for cooking meat is described in Jacqui Wood's book, Prehistoric Cooking, along with Paul Elliott's book, Food and Farming in Prehistoric Britain. I kneaded up a simple dough of flour and water. I covered a flat cut of lamb with mint leaves and wrapped the dough around the meat, sealing it completely.
It was a damp December day, so drying the dough took many hours. Placing the bundle on top of the cook stone dried the bottom. Then I tried to dry the other side by flipping the bundle over, but the difference in rigidity between the dried and not-dried halves meant that the dough cracked. It's preferable for the packet to remain sealed, to keep in all the delicious juices, so I do not recommend trying to flip your packet over.
Wood and Elliott describe cooking the packet in an earth oven similar to that used in Hawaiian Luaus, where a fire is built in a pit lined with stones. The stones are heated, then scraped clean(ish) and the items to be cooked are placed on the hot stones. More hot stones are piled on top, the pit is covered with soil, and the contents bake for several hours. Instead of a pit oven, I made a space in my fire pit enclosed with stones, and heated the stones with fire from the inside. Once the stones were heated, I scraped out the coals, placed the packet in my makeshift "oven," covering it with a flat piece of slate and piling coals on top.
After two hours the lamb was perfectly done, moist and juicy. If I had not broken the seal of the dough earlier, I would have been able to turn the packet over and use the dough as a bowl for serving. Because of the cracks from when I tried to turn it over while drying, the juices wanted to leak out, so I put the packet in a dish. The dough in this dish is not really edible, and it feels like a waste of flour--but the meat was incredibly delicious! This is more likely to have been a feast-day dish to feed a crowd, rather than a regular daily meal.