Paul,
 
I read all of the stuff on removing odors with some interest as we have
all faced this problem.  I recommend the following:  1) soak the shell
in a bleach mixture with the aperture up (24 hours - except for land
snails), 2) wash it out with water pressure and be careful not to drop
it, 3) let it drain aperture down and repeat if the odor is still there,
4) when dry fill the shell with alcohol (again aperture up) and let sit
over night.  If there is still too much smell (frankly I no longer mind
it but my family does not agree) as a last resort I put a spoonful of
baking soda in the aperture and then seal it with cotton.   A lingering
smell means there is still some of the animal inside, rotting away.
Sometimes you just cannot remove all of the inside stuff.  You must be
careful to make sure it dries up or it can really discolor a shell,  The
alcohol does a lot to dry it up.  If you finally resort to the baking
soda and cotton, I recommend cleaning it out after a year or so.  Good
luck,
 
Tom Eichhorst in New mexico, USA