My friend used to make birthday cakes and sell them, a trick I learned from her was to use cardboard boxes from the supermarket, cut a board the size of your cake(s) cover with foil. Place cakes on top of this, on another box that she would open up to the bare shape, cut the sides to just above the height of your cakes. Tape the corners, so that you have a custom sized box around your cakes, then seal the whole lot with plastic wrap.