I made the cake for our Grandsons Christening. The hot air balloon is hollow, made from gumpaste, molded from an Easter egg packaging that I had kept. My husband goes crazy about all the bits and pieces I keep ! I cut a round from thickish cardboard to mount the balloon onto and glued the wooden skewers into this. Once dried, I covered the round with gumpaste and once into position secured the balloon with a gumpaste strip and royal icing. It turned out to be very secure.