I work in a prison and i think that when i see the cons being moved in and out in them!! I'll have to ask G4 and see what they do with them!! Not sure about the strength with insides removed though.......
We looked into this and as jrp204 says they're not high enough internally for horses but would be ok for ponies, as apparently the floor insise is raised or something. Shame though they'd make nice horseboxes!