Hi Jo; Welcome to permies!
If I understand correctly, your currently building a fire inside that tank to heat the sweat lodge rocks piled inside ? You would like not to stick your head in a smoke filled box to retrieve hot rocks ? :)
You are thinking about how to use a
rmh to do that ? It
think can be done!
You need a rocket core and barrel. Your
rmh fire built with small dry wood will be super hot with no smoke. The hot air from the 55 gallon barrel is pushed by convection into your current tank,and then vents thru your 6" stack. You would want to seal up all your vents at the bottom. A several inch thick layer of insulated
cob on the bottom and up the sides. Leave your 6" chimney and expect exhaust leakage at your rock door (as its outside this matters not)
Built properly your rocks
should heat up in record time, I can promise you WILL use way less wood . You will NOT smoke out the neighborhood.
You will want a copy of the builders guide (Ernie & Erica Wisner) to learn how to properly build a rmh.
I think you would want a brick manifold behind your core to plumb the hot air into your "bell"
Think about it... do some research.. come back here or over at the
rocket stove forum and ask questions !
Welcome to the wonderful world of rocket science !