This is the bottomless chicken pen that we built from the aluminum frame of an old window awning. It has a floor space of 30 square feet. I have put six hens in it and moved it around the yard this fall, effectively taming an area that was knee high in lambs quarter and ragweed. It takes about two days for the hens to eat all the vegetation and scratch it up. On the second day I've been feeding them a mixture of oats and buckwheat to "plant" a winter cover crop. They miss enough oats that I've got decent coverage on the places they've already been.
The pen is heavier than it was supposed to be, because of the very large and sturdy nesting box. That part detaches and will probably be re-engineered over the winter. The hens were very bad at foraging at first; they just stood at the wire and begged for grain. Over time, they turned more industrious. Now they are scratching like champs and even laying eggs despite a minimal diet and no lights.
Looking for more chicken pen ideas? Try the City Chicken's chicken tractor page.