jacob wrote:
What about a traditional turnkey latch - piece of wood with hole in
middle nailed to door frame (or screwed with a cup washer if you want
to be posh) which you turn from vertical to horizontal to keep the
door shut.

I could do that but I'd need to make additions to the door and/or
sides. Currently the door is simply a sheet of 1/2" plywood hinged to
one edge of a box also made of 1/2" plywood. One of the aims of my
design was to keep the inside as smooth as possible to easy cleaning.
I suppose I could arrange the above all outside the henhouse.

--
Chris Green