I really don't know what to do. I finished making my son's birthday cake (cupcake cake), however it's really long (28 inches) and I don't have a box that big to protect it in the car - we're having the party at a place about 45 minutes away. How can I cover it and protect it during that time?
You can take a box apart and rebend the sides to fit your cake. Staple or tape it back together. It doesn't need a lid unless you are planning on having something fall on it ;-) (or if you let your kids throw things..) To protect from road dust, use plastic wrap.
I do guitar cakes quite often that are life size, I never put them in boxes, just the back of my van.
When I deliver cakes that size I don't cover them. Just make sure the car is clean and nothing will fall on it and damage it, and you will be good to go.