I mentioned in this recent post that I was working on a method to measure the volumes of snail shells and that I needed a way to seal their apertures against water. After tape, chewing gum and plasticine failed, my wife came to the rescue and suggested silicone ear plugs. I tried them tonight and got good results. The shell in the picture was sealed with silicone. The main drawback is that the application and removal of silicone is time consuming.
Results are forthcoming.