It is always good to plan ahead, so you are not trying to work this out the day you want to move the cake.
When making Vickies cake I knew that I wanted to make each tier at last 4 inches high and there would be decorations on top as well, so I had a rough height for the cake in mind.
Storage busineses are a great place to source boxes. For this cake I purchased a 1/2 tea chest from the Storage King in Greensbrorough. Sue Dimovski was very helpful, finding me the best box to protect the cake. I then made some alternations to the box. There is less risk of damaging the cake on the side of the box if it can be slid out the side, rather than lifted out of the top, so I simply used a blade to cut two sides of the box. The sting hanging down is what is used to tie the side back up. The top corners of the box were all tied together to prevent any movement and the top was covered with paper and taped in position.
So there you are, a safe way to transport a large cake that protects it both from damage and contamination.