I made a spider cake once doing something similar. I made two round cakes, placed the bottom layer on my cake board, cut out a 3" circle from the center. Poured not quite set green jello in the hole. Then placed my second cake on top (this one was baked in a stainless steel bowl so the entire thing looked like a dome when stacked) Ten I put a smaller bowl-shaped cake next to it and frosted with black icing, used black licorice for legs, and cinnamon discs for eyes. It was a big black spider that oozed green goo when you cut into it.