The chimney we created as we built up the arch.    Essentially as we put the front brick vestibule on – we subsequently added layers of cob that connected the arch and the oven – but we left a hole (chimney) at that joining point at the back of the arch.   You can build a clay chimney here – or if you want the smoke to be taken up up up and away (out of your face height) you can install a longer flue (not galvanised!) which we did.    We put it up through a roof and put a chimney top on it keep the rain out.