My Friend Greg was the lead builder on this. We started with recycled plywood crate for $15. It's 43" X 43" by 47" high. we cut openings, covered edges with 1X2 boards. added wheels, slanted roof with ventilation but secured with 14 gauge, 1X1 welded wire like the cage . then built the cage to fit over the pop door.. roost inside and a 5 gallon bucket will be a nest box. have to cut the welded wire away from the pop door and paint the cage wood yet.
also will put a vinyl floor in the bottom (removable) for easier cleaning. I can crawl through the back door if I need to, but mainly its for gathering eggs and putting in feed and water.
I wish we'd found a way to make the cage go under the coop and used bigger wheels, it's heavy and awkward to move around the yard, but it's strong and secure.
if predators are too persistant, I'll add a welded wire 'skirt' to attach to the cage bottom around the outside against the ground.