I would use royal icing as it dries hard. You can buy the powered sugar from the supermarket (I think you just have to add water) or make your own using ordinary icing sugar, egg whites and lemon juice.
Here's Mary Barry's recipe, but this is to cover a 9 inch cake, so you would likely only need a third or half of the quantity depending on your design.
2 egg whites 500g sifted icing sugar 4 tsp of lemon juice
Whisk egg whites with a fork until bubbles form on surface Add half icing sugar and lemon juice and beat until brilliantly white Gradually stir in remainder of icing sugar until desire consistency