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.
Сирень из бисера. Схемы
Brooch with crystal, pearls and sequins.
I don't read Russian, but I know a beautiful bullion stitch when I see one! I wonder what the raised base is made of.