Not aware of any 28mm Roman chicken coops (Gallinaria) but there are a couple of descriptions of how to design coops by Roman authors. They may help you find something suitable which was not designed as a Roman hen house.
This is from
http://www.aviculture-europe.nl/nummers/09e03a11.pdfGallinaria - housing in Roman timesLet me quote Columnella:
“…… Poultry yards should be constructed in that part of the country estate which
faces the rising sun in winter; they should adjoin the bake-house or the kitchen
so that the birds may be reached by the smoke which is very good for them. The
structure, that is to say the henhouse, should consist of three adjacent
chambers, and the entire front, as I have just said, should face east. In this front
there should be only one narrow entrance to the middle chamber; and this
chamber, which is the smallest of the three, should be seven feet in height and
in each direction.
…. Birds should not be allowed to sleep on the floors, lest they be harmed by
their droppings, which are apt to cause gout if clogged under the arches of their
feet. In order not to incur this disease, smooth, round perches are fashioned that
do no harm to the birds that settle on them. The perches are fixed in holes in the
opposite walls in such a way that they are a foot above the platform and there
are two feet between them…..”
Varro stated as follows:
“…If you wish to rear two hundred fowls, you must prepare an enclosure containing two large connected
hutches facing east, each about 10 feet long by 5 feet broad, and rather less than 10 feet high.Each hutch should have a window three feet by one and these should be protected with osier twigsspaced in such a way that light may enter without, however, allowing any noxious beast to
gain access and harm the chickens…..”