I apologize in advance for using the wrong terms. I haven't been able to find pictures or read descriptions about how the ducts in the mass part of the RMH actually connect to the steel drum, where all the heat comes from. Is it so painfully obvious to everyone else that it never needs mention? Where can I find this info?
Good question, to be thinking about that area. Many people don't think about the transition.
It is commonly known as the "manifold", and should be shaped so that air can flow smoothly from the ring inside the barrel to the duct. As there are numerous possible orientations of the duct, there can be no one best manifold design, but in general you want the beginning of the manifold at least two times the cross sectional area of the duct, tapering smoothly to the duct size and shape. A search on "manifold" should get you lots of discussion of this area.
Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
Karl, one straight forward way to construct the manifold is by using a second matching barrel, cut approximately in half, to serve both as the exhaust connection point and as a base for the full barrel. The full barrel is band-clamped on top of the 1/2 barrel, using an appropriate seal (like fiberglass wood stove door gasket rope held in place with high temperature aluminum tape, and the barrel's band clamp over that) . There are lots of photos of such beginning on this page: