I was involved in the building of a small clay oven in Grizedale forest in the Lakes.
The clay was dug out of the stream bed.It worked really well and lasted over a year
until some gallumphing woozle sat on it and trashed it completely!We replaced it
with a brick one which was still there last time I looked.A friend up here has a hogback oven
made with waste from a pottery over a willow frame which has been going strong for at
least seven or eight years , but he does have a galvanised "roof" over it to stop the rain
getting at it.Easiest pattern seems to be the cob oven , which looks very easy to construct
and should last for some years if protected properly.
Go for it , and post pics. Cheers , Simon