It was to be able to easily remove the firebox. I am planning a larger smoker build and I wanted to try using a modular firebox setup. Mostly to see how well it'd work, but it also saves time and material when fabricating.Nice! I notice the air gaps in the corners where it slides into the kettle. Was that intentional? After watching your first cook and thinkin' about it a bit, I'd say those "air gaps" might help keep temps more manageable. Allows some air flow into the kettle that's not wood-fire heated.
Red