So here is the finished "tractor". The term comes from the use of chickens to cultivate and fertilize the ground below. That's not exactly our intent, but it is nice to let the chicks mow the grass. Since we've been using it, it seems a move once a week is about right to keep the grass green and the girls happy. It's not super easy to move, but do-able with the wheels on the left above.
At about two months, our pullets are fully feathered and seem to be very happy about being outside. They took to foraging with no trouble and are now enjoying a diet of pullet developer crumble plus plenty of grass and bugs.