I built myself a piezo hydrophone. The metal ring used was kept from a disassembled camera lens. I sealed around the edges to make a box before casting the whole thing into resin. 

Resin Tip: If you have a lighter handy you can quickly brush the flame over the surface to pop the bubbles during initial casting to avoid the bubbles seen in the bottom photo.  😉  But in this case I did not have one handy before the resin setup. Far as I can tell keeping them does not hurt the hydrophones function since it’s active side faces outward.