Fill nylon stocking with concrete for the base. Create gills underneath the cap with thin sheets of metal. Sculpt a mold for the cap, and press it into WET sand lined with a thin sheet of plastic, and slowly fill with concrete. Add realism by removing small wedges of concrete along the edge of the cap. Cap mold: A circle of cloth sheeting could be draped over a bowl/balloon, and brushed with concrete. The wavy edges could be created with rolled up newspaper under the edge of the cloth.