As with lots of questions, the 'real' answer begins with "It depends..."
 
If you're out picking up Cassis cornuta, then a plastic 5-gallon
bucket should get you by for a while...
 
But, in my experience with 'standard-sized' shells taught me to use
a round, Rubbermaid(r)-style refrigerator container. You can CAREFULLY
put a small hole through the outer tab of the lid, and a second small
hole through the outer rim so that a small Nylon(r) line can be
threaded through them and tied in a loop. This allows you attach the
collecting container to you bouyancy vest, leaving your hand(s) free
for the things hands need to be free for.
 
I know, "What a hassle taking the lid off, putting the lid on, taking
the lid off, putting the lid on,..." The "trick" is to use a very
sharp, thin blade (I use X-acto(r) brand -- or suitable ersatz) to
make two cuts through the center of the lid, at right angle to each
other. Leave about 10% of the diameter at the outer edge of the lid
uncut (so the lid stays fairly strong).
 
That's it! Now you have a container that you can 'pop' a shell into
with one hand (gloved, even) and not have to look at it. You get a
good, definite "thup!" (that's the underwater equivalent of "pop!")
as your shell (anything from a 1/4" Cypraea fimbriata to a 3+ inch
Cypraea leviathan) goes where you want it. (Did I mention that I'm
mostly interested in cowries?)
 
A nice thing about this container idea is that it's rather easy to scale
it up or down in size for whatever you're collecting... within reason,
of course.
 
Happy shelling!
 
Aloha,
 
makuabob (a.k.a. Bob Dayle)