We started with a small coop for our first three leghorns but when we got more chickens we decided to modify a lightweight greenhouse we had gotten a while back (I think from Ebay). Here is what that frame looked like when the old cover was still on the back. You can see our first coop in the background
We fastened it to a frame of pressure treated wood I had built on the ground and covered the area with weed blocking cloth. Some wood support was added for a door frame and hardware cloth fastened to the whole structure and swooped out on the bottom and covered with soil so that predators can't dig in.
We covered it with tarps which were pulled open for ventilation in the Texas summer and fastened down in the winter
The throw bolt lock has a bolt dropped into the padlock hole to foil clever racoons that we read can actually open a throw bolt!
This arrangement worked well for a while but the Texas sun is murder on tarps and they were basically rotting away so we decided to upgrade to corrugated polycarbonate roofing (8 foot panels I cut in half see http://www.homedepot.com/h_d1/N-5yc1v/R-100025674/h_d2/ProductDisplay?catalogId=10053&langId=-1&keyword=Suntop+roofing+panel&storeId=10051 ).
Problem was I didn't want to take on the cost and labor involved with building it at the strength level of say a shed, but keeping the new roof structure minimal meant that I wouldn't be able to walk on it to fasten the screws. So the roof was built on the ground and the I fastened some 2X4X12s to our garden to act as a ramp to that my wife and I could slide it up into place. Support posts and beams were built on the sides of the coop to handle the weight.
Finally we added plywood panels to the front to block winter wind and summer sun. On the sides tarps were added which can be easily opened or removed in summer for added ventilation. We're hoping the tarps will last better on the sides as they won't be getting blasted by the sun on the roof and will probably be packed away when it's warm out. Final touch for fun was a Foghorn Leghorn decal decoupaged with spar varnish.
Well the chickens seem to be happy -- now back to work on the human house!