If you don't have the candy bar we used for the trap door, you can use decorating icing or gel to draw it on the cake.
Look for the bubble gum-flavored soda with the other soda pop.
Paste food color will produce more vividly colored frosting.
Create an attractive display by covering a piece of sturdy cardboard with wrapping paper, then plastic food wrap. Stretch and secure with tape. Or cover cardboard with foil or cooking parchment paper.