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.
Sally sells sea shells by the sea shore. She sells sea shells on the sea shore; The shells that she sells are sea shells I'm sure. So if she sells sea shells on the sea shore, I'm sure that the shells are sea shore shells.