I put together this seashell shadow box for my beachy bathroom recently.
I purchased four shadow boxes from a dollar store and a bag of various shells.
I glued the shadow boxes together and then painted them with acrylic paint in a creamy colour.
I mixed up some sand I snagged from a playground, and mixed it with invisible school glue and more of the cream acrylic paint.
I carefully applied the sand mixture using a small paint brush to the backs of each shadow box and after it dried, glued the shells to the sandy paint using a glue gun.