Icing - what kind best for a cake that will stay in a car all day?

[HarriedWithChildren]

Could anyone tell me what they would recommend icing a birthday cake with, knowing it will stay in the car all day until we get to our destination in the evening?

• It's going to be a sponge of some sort
• Will have boy theme & shape, e.g. castle, rocket...
• The car will be mostly cool at this time of year and will be in boot so no overheating

The icing doesn't have to be perfect but I know marzipan goes all shiny and wet looking in the fridge so keen to avoid that. I was thinking of using roll out or ready rolled sugar icing and then painting it with food colouring. Would that work? Butter cream better?

All advice/recipes welcome, many thanks

butter cream underneath and then fondant /sugar iciong on top.
I made my mum a cake and took it to her in the (hot) car for 4 hours and it was fine.
I took decorations to add on top in a separate box and then stuck them on once we got to hers with some water.

All righty will try that, thank you so much (sorry about delay, computer problem). Sounds yummy as well as can't abide mega thick, rock hard icing. Have you ever tried painting icing with food colouring?