By the looks this guy is really into his oven(s) & operates the site on a donations via paypal principle then sends you his pictoral / technical cd from Oz
theres plenty of guidance as is.
http://www.traditionaloven.com/couldn't find the thread discussing this sort of thing from way back so new thread & source.
Noticed someone is selling a fibreglass reinforced cement version (here in the uk) thats moulded over an inflated sack to start the shaping process.. (pricey though)
http://www.dingley-dell.com/home