I found 2 by a dumpster plus an old wald wire front mount basket.

Just gotta know where to look & who to ask. They are like shipping containers and are often abandoned when a company goes under that used them or switches to a new crate design. Ive seen them in garbage sorting centers 3x (field trip when i was a kid).

Have you thought about getting a rectangular crate, attach it so the narrow part is behind the seat, keeps the weight from being too far back.

- Andy