As a precaution to minimize damage, for a full size stunter, I would recommend 1/4" ply construction with 3/4 x 1 1/2 external frame, using plenty of screws. The frame helps in handling the thing. Also, recommend a supports in the center of the box to minimize crushing if a heavy package is placed on top of it, mark top and bottom and set it up so that it is clear how the thing should be positioned, not that it would make a lot of difference. Make it clear that a model airplane is inside and is to be handled with care. However, to the handlers, it is a box and will be packed with other packages in the easiest manner.
Keith