I used a mylar bag and properly sealed the top with tape. Then, the book inside the mylar bag w/ backing board were taped securely to one of the rigid pieces of cardboard, exactly like the first pic of step 3 in your post. Next, I placed another rigid sheet on top of that and taped them tight, sandwiching the comic/backing board together between the rigid sheets.The two rigid sheets of cardboard were VERY tightly taped using packing tape - I did this to ensure the book wouldn't be floating around between the cardboard sheets. I knew the box would likely get tossed around during shipment, therefore I made absolutely sure the book would not be shifting around at all. Should have taken pictures for documentation as I packed it up, but hindsight is 20/20. I extensively researched how to ship valuable comics to make sure I properly prepared the book for shipping