I use powdered laundry detergent (Charlie's), Biz or Oxi-Clean, and sometimes washing soda. At least the first two, or all together.
The smell should probably not survive a long soak in that, and an afternoon in the sun is bound to help.
Beyond that, look for an enzyme-based odor eliminator, that is almost always effective, but good ones are hard to find outside of industrial scenarios. United Laboratories makes a great one, #455, but individuals can't buy it direct.
