Thanks, I got a good idea of it's size now. Well, after I converted your numbers to inches, that is! Hahaha!The size is about the same as a 13kg gasbottle. Max. diameter on top is about 33cm, height is about 50cm. Later I could take the exact messures and post an additional image.
we have done something similar, we have pellet stoves, and definitely of course running the vertical chimney inside gives extra radiant off it. We also put vertical running 10mm fins, spaced around the 80mm vent, 20 fins total. Some people say that it even works better if you put the rings horizontally like washers, as that creates more turbulence, and helps keep air caught longer against the fins to heat more, but I don't know if that works. Ours, after putting the fins on worked noticeably warmer in the room.