After a brief survey of possible designs I settled on a 4' x 4' x 10' frame of ½" EMT secured by elbows and bolted corner braces. I used copious black zip ties to attach panels of 2" x 4" weldmesh fencing. The roost consisted of a length of 2" x 3" sheltered inside a recycled plastic barrel. The ladder proved unnecessary: the hens ignored it and happily jumped up to the roost. A tarpaulin completed the defences against the elements and provided a measure of camouflage. The total budget was well under $100.
After some months of use some drawbacks have become apparent. The tractor is light enough that it can be moved by a single person — an easy way is to prop a kid's scooter under each end. But it can blow away in a strong wind and requires tethering (I used tent pegs). A more serious problem was its defences against predators. A 6" apron of weldmesh around the inner circumference proved sufficient to prevent foxes from digging underneath. Sadly I had not reckoned on raccoons being able to reach inside the cage and attack the hens directly. Reinforcing the mesh walls with ½" hardware cloth might have prevented the loss of two hens.
More recently, the cage was demolished by snow! The structure just could not withstand the weight, and I will need to replace the frame. In future I will take off the tarpaulin whenever snow is forecast.