The N Gauge Society website has free downloadaable paper container designs, shown in the above photo. There is now no accompanying info about the prototypes but they were used from the 1960s (Cunard is the earliest type) into the early 1980s.
https://ngaugesociety.org/wp-content/uploads/Tartan_Arrow_20ft_30ft_Containers.pdf
https://ngaugesociety.org/wp-content/uploads/Winnic-30-Container-Assembly.pdf
https://ngaugesociety.org/wp-content/uploads/Cunard-20ft-Container.pdf
https://ngaugesociety.org/wp-content/uploads/Rootes_Container_Sheet_A.pdf
https://ngaugesociety.org/wp-content/uploads/Rootes_Container_Sheet_B.pdf (Chrysler branded)
These blocks are designed so you can make up one of each container design, and have it be sturdier than if it was just modelled in card, which was the original media for the kits.