This is what I do.
pack your cake as normal. whe you go to close the lid, tape the sides down on the box so that it does not close onto the cake. so with the sides of the cake taped to the box to keep it propped open, measure a piece of plastic wrap long enough just to wrap around the sides slightly so that the open area is covered. Then I tape the sides of the plastic wrap down around the box.....I do this because I don't want anything to get into the box during delivery, since it is taped open..........
For a less pretty method, you can get a 14X30 brown shipping box from walmart for 64 cent and pack the cake in it.........they have one that I use when traveling with a cake....i think..
hope it helps...good luck