You talking about the metal drum? Seems like that’s necessary. He explains why in the video, tldw: it radiates heat which cools the exhaust, causing it to sink, and this starts pulling air through the combustion chamber. He calls it an air siphon at one point. Yeah you lose some heat from this, but without that siphon it won’t create the jet furnace in the firebox. Maybe there’s a way to engineer that without metal, I have no idea.
Good point!