Celebrations tubs are perfect for keeping cake fresh. If you have Tupperware, use that instead - it makes no difference to the functionality of the finished result as long as the cakes are a reasonably good fit in the container.
The piece of oak are nailed into a slab of 1/4" MDF board in four opposite areas around the tubs (on a clock face, they're positioned at 12, 3, 6 and 9 o'clock.) I ensured they are a tight fit against the containers, so that they cannot move at all.
I drilled holes in the corners and centre points of each edge in case the board needed to be tied off or held in place with bungee cords in order to prevent movement. However, this turned out not to be necessary.
Finally, I added a layer of 1/4" upholsterer's foam to the underside of the board. On the carpeted parcel shelf in the car, this completely stopped the board from moving around.
It's not fancy, but it works well, so I figured it's a good thing to put up here for my first Instructable guide. Thanks for viewing!