I am making a small stacked book cake and need to drive it 8 hours. What's the best way to do this?
Make sure you have dowelled it (or used straws or SPS) properly, then put non-slip matting between the cake board and the box.
Make sure the box is as flat as possible in the car, with nothing around or above it that can squash it if you brake suddenly.
The aircon should be going, and something to cover the cake to shield it from direct sunlight. Either a lid on the box, or a shade on your windows.
Don't stop in the sun for lunch, and drive carefully!