put small christmas shape made out of cake on side of a 10in cake made out of BC. I want to make it look 3D. I had thought about using toothpick to hold it up and push it into the BC icing to help it stay in place. The cake is going to be 4in high if that will help.

royal icing dries hard as rock. Maybe try that, and use toothpicks to hold it in place until the icing dries, then remove the toothpicks.

Melted chocolate would also work.