Model submarine packing and shipping can be a big PITA if done improperly.
I build my own cardboard containers by cutting flat sheets of cardboard out of refrigerator boxes.
Check with the Big Box stores for free cardboard.
I use a long metal straightedge for bending the cardboard into a one-piece carton and then cut two cardboard pieces for the ends. This makes a much stronger carton then cutting individual pieces.
The shipping carton is made being 3" oversize on all sides.
The submarine accessaries are bubble wrapped and inserted inside the hull. Any extra space is filled with bubble wrap. You want to keep anything from shifting inside the hull and causing damage.
Wrap the entire hull in heavy duty trash bag or bags, depending on how long the hull is.
Then lay down some expandable foam sealant on the inside bottom of the carton and carefully lay the hull on top of the layer of foam. Leave space on each side of the hull and fill those side with more expandable foam. Have a spray bottle with water ready to spray onto the foam to help accelerate the curing. Now continue to lay down foam onto the top section of the interior of the carton. Let the foam cure for an hour then trim foam, if needed, and tape the carton up and you are ready to ship. Dont go cazy with adding too much foam. it can cause too much expansion and distort to shape of the hull.You just want to embrace the hull and keep it from becoming a torpedo. This may not be suitable for customs inspections because of the the foam incapsulating the hull but it will arrive undamaged, otherwise.
You may want to use a large Sharpie pen and write do not stack boxes on this box and fragile all over the surface.