I built ours an ark using a sheet of 8x4 sterlingboard as the floor and two sheets of marine ply (same size for the pitched roof. The whole thing was mounted on three runners.
They've pretty much outgrown it, not to mention wrecked it, so I'm going to build them a new house before the snow comes, along the same lines as the goat house I built, but with a raised floor for warmth. Costs a fraction of the price of even the cheapest pre-made sheds.