Open the top box! Make the box up, with the lid open. Place the cake inside, then wrap cellophane around to’seal’ the box. There is no need to purchase a taller box unless it is explicitly required. I know that sounds bizarre, but a cake maker did this for us, and it worked well.
@Caleb I don’t know whether anyone responded, but box extenders are available at various retailers. They function as a piece of additional, bottomless box to fit in between your existing ones and make it rallee.
@Linny Nope, not at my usual bulk barn.
I’m aware that packaging made especially for tall cakes were sold for two years prior to the pandemic, but those sales ceased. In addition, cake boxes are becoming absurdly expensive. They were two dollars before, and they are now five dollars.