Like Marcus, I have had my experience with a smelly Cassis tuberosa.  After
trying Clorox soaks for days on end, time after time, I finally tried baking
soda.  I poured almost a full box of the stuff into the shell with a lot of
shaking to get it as deep as possible.  Then I completely filled the
aperture with cotton - stuffed very tightly.  The shell was then put in the
cabinet and no smell!  After six months or so, I removed the cotton, rinsed
out the shell and put it back in the cabinet.  Bang!  The smell was there
again.  So, another box of baking soda for another six months (actually
closer to a year).  This time it seemed okay when I cleaned it out.  Of
course, most of us can tolerate a pretty high level of such a smell.  My
wife swears I enjoy it.

Tom Eichhorst in New Mexico, USA