I like the idea of painting the bra to look like your desired seashell instead of adhering one to the cup. May be a good solution for making sure things stay in their places over the course of the day.
"The Birth of NOLEMIRE THE MERMAID " Steel Corset by FIORI Couture. One of a kind gold brushed steel corset embellished with Glass Beads and Swarovski Crystals. It might hurt a little to wear this, but I would combine it with a long, white skirt.