Cob layers around a small rocket stove

I have some red clay chimney flue I've cut for a small rocket stove to boil maple sap.  I have done some searching (I promise I have!)but still have some hesitancies about the type cob to use.  I have at my disposal clay, perlite, refractory cement and straw if need be.  I had trouble with a large stove that shrunk and cracked when I used perlite:refractory 4:1.  I guess my questions would be how many layers, how thick, what materials in what proportions.  Thanks in advance.