Normally, I also use a small net bag as I have a rule: "No big shells".
 
This suffices for most shells, and I rarely lose any.  However for very
small shells close to the net mesh size, I also have a plastic
35 mm film container attached to the bag by a string that passes through
the cover and through the container bottom terminated by an object too
large to pass back through the holes.  I always have pleanty of film
containers as taking photos is why I go on trips.  Therefore, I can
always make up a new one.
 
Aloha,
 
Wes