DAP

Sometimes you just need to carry a large conch shell or some other object like that on your boat. Left to its own devices, it will surely fly all over the boat creating mayhem. Get DAP, also known as museum putty, at your local hardware store. Put a wad on the bottom and press item onto the flat surface where you want it. Chances are it will stay put. Experiment and watch it at the beginning until you are confident that it will stay put.