ETA: Dad called them Grandpa bins. After we lost a horse to some molded grain we put galvanized trash cans in the spaces inside so they could be removed and sanitized. He capped the corners along the bottom with metal to keep the mice from chewing into it. He used decorative nail heads to secure the metal, some nice hinges, old leather harness piece for a handle, and sanded it and stained it to match the stall fronts.
Inside, the shelf folded up on hinges and a chain on each end kept it from falling all the way down. The casters were heavy duty with the lock down tabs on them. They swiveled so it was really easy to unlock, pull out and sweep behind.