We are new to building mass rocket heater but have read most of the books and watched a lot of you tube. Now we are getting ready to build this spring. Our question is of course on the manifold. We are planning to use a cut down 55 gallon barrel for the manifold, with a whole 55 gallon on top. We have read where the manifold exhaust exit to the channel should be a least 2X that of the CSA of the burn tunnel. We will use 8” 26 gauge galvanized steel duct pipe for the channel and thinking of using a 12” X 8” 26 gauge galvanized steel duct pipe reducer to connect the manifold to the channel and coat the hole thing with a couple of inches of a mixture of high temp. clay and sand. The 12” end of the reducer gives you a CSA slightly more than double the 52” CSA of the 8” channel pipe. Any thoughts?