I thought the idea of a portable "tractor" to move the girls for fresh grass and sunshine was a great idea! We had let them free range one day but we were so nervous about it that this seems to be the best solution.
So we kept it simple.
6 ft by 6 ft by 2 ft high covered on top and sides in 1 " x 24" poultry netting
2x2's for the frame
Training wheels that had come off the kids old bikes
a thick nylon rope to pull it to and fro and a couple of trim pieces for the door hatch which just hooks on to two screws to close
They LOVE it and it works great!