Ava Zim, this sounds like a "fun" project! I would like to add a few more thoughts. When the wood in the box is heated, the first component to appear in the output is going to be water vapor, followed by steam, and then a mix of smoke and steam, maybe some wood vinegar, and finally smoke and more smoke, eventually making a sound like a jet engine, and tremendous amount of heat. (I know this because I once built such a device.) When this output gas is lit and is hot, you will be able to reduce the output of the gasifier or waste oil burner, because the output from the inner box will make plenty of heat. In my experiment, I let these flaming gases under pressure blow onto some lava rocks just above where I introduced the external air. It is a very NON-LINEAR process! The gasifier blower should be hooked to a device to vary the speed so the output of the gasifier can be controlled somewhat. Remember to start the gasifier with a good load of well-made charcoal, if the plan is to use a gasifier that uses chunks of DRY wood. Once the gasifier is up to temperature, it will convert the wood to charcoal, but there must be charcoal for the gasifier to work.
One method of determining when the wood in the inner container is done is to listen to see if the "jet engine" sound has stopped, or actually look at the gas exiting the box into the combustion chamber. The wood in the inner box must be uniform in size, else some will be converted to charcoal, and the larger hunks will only be torrified.
The end result should be charcoal that will ring like a bell. Be sure to store it in an airtight container. One reason for doing this is to keep it dry, but the main reason is to prevent it from self-igniting and burning itself to ash.