When the rainbow birthday cakes was cool I frosted only the top and then cut it in half and then stood the two halves up together. I cut wooden skewers and put four or five of them clear through the rainbow birthday cakes designs to make sure it stayed together. I put a thin coat of frosting over the whole thing to seal in the crumbs. I left it overnight and frosted it the next day. So, make your own rainbow birthday cakes ideas.