When we were hauling wood chips on the DM&E, they were carried in shipper-owned 40' box cars. The inside of the door openings were covered with "grain doors". A grain door is a wide sheet of heavy corrugated cardboard, backed on the inside by several 1x6" planks running longways in the door opening.
The original purpose of grain doors was to seal boxcars for loading loose grain.
For paper, it's a combination of polyester web strapping, steel banding, air bags and sometimes cardboard dunnage/blocking on the floor to keep rolls from shifting. Grain doors would not be used in P&P applications - the goal is for the blocking and bracing to prevent load shift - especially against a door.
Google Image "boxcar paper load" and several pics will pop up with examples.