Thinking out of the box a little, with metal skips there are often rust holes and lots of bends where loads have been carelessly dropped in the wagons and JCB loading have done side swipes. I would have thought this is just the thing for 3d printing to achieve, possibly drawing up a few distressed squares to insert into the plain sides at the drawing stage. On my LGB ones I attacked the sides with a soldering iron, cant show these now as they have been sold out of service.