I think I would try beading the design on before cutting it out. I could still cut the individual curved designs between each section of the shell as shown and stitch them on then continue beading.
Crafts with their hands - beautiful brooch of beads and beads
Brooch with crystal, pearls and sequins.
Brooch with a crystal, pearls and paillettes. A master class - the Fair of Masters - handwork, handmade // Martha Foss