When I began my thought experiment, the presence of maize seemed very unlikely in view of the known dynamics of rainforest soils. Yet I needed a plant that produced packable waste that could be handled without steel tools. Wood was both high cost in human energy inputs and very resistant to charring and crushing. Most other crops simply failed to produce both a crop and much biomass. No primitive farmer was going to plant a stand-alone char feedstock and lose a season.
This is where corn or maize came in. It produced a stable easy to store high volume crop that also produced perhaps ten tons per acre of corn stover. This stover was also very packable because there are no branches. What made it more attractive was the root ball which is in the form of a disc and is often very easy to pull out of the soil. Thus a field could be stripped of its ripe corn and then stripped easily of its stover.
Stacking the stalks was easily accomplished and using the root balls to form an outer wall simply a matter of paying attention. The key idea was to provide an outer shell of mud that closed off the packed stover. Now they did not have a sheet of metal foil to add another heat resistant air tight layer, so it is likely that they slathered on a thin layer of river clay to form a air tight seal. Again field experiments will inform us as to the extent that this is all necessary.
At the end of the day, without any tools, we have a thin clay dome or a mud dome enclosing ten tons of packed stover.
This is then loaded with a charge of burning coals. I have considered top down but suspect that simply feeding a charge in through the bottom perhaps along a narrow trench will be good enough. A small amount of air will be drawn to the charge maintaining the heat production and the produced heat will steadily reduce the maize very quickly. Gasses will be captured and ignite as the burn progresses steadily reducing the load.
Eventually the whole load will collapse upon which it will be smothered with more dirt.
I had originally envisaged this process taking many hours, however corn stover is like paper and merely needs to be heated for it to curl up and quickly decompose.
Ten tons or one acres production would give us three tons of biochar which is ample for that one acre, particularly if one goes the extra step of creating seed hills on only a third of the surface. In one season, you are in business. The one remaining mystery is why this method failed to make it out of the Amazon, because it would have nicely augmented the three sisters throughout the Americas. Or perhaps it did and we simply never noticed or our steel got there first and disease got there first.