I would make sure the cake has enough support for the upper layer and a dowel down the middle of the cake through both layers first. I started using brand new cardboard moving boxes to transport cakes. I get the size that's the same as the cake board that I'm using. I cut two sides of one of the box panels so it opens up and I can slide the cake into the box. Then I tape the panel back up with duct tape or clear moving tape. The cake doesn't slide around in the box since it fits snug and the cardboard is stronger than any cake box I've found. I then put a towel with a drawer lining mat under the box in the car so it absorbs bumps in the road and doesn't slide around. I hope this helps!
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