Have you tried soaking it in an enzymatic cleaner, even something like Nature's Miracle for animal "accidents" in the home?
I use Lestoil for our really stinky stuff, which might be worth a try.
In the good weather, I'm a big believer in hanging stuff out in the sun. The UV rays are great natural odor killers.
We use cloth diapers, and for a really stinky batch, I soak in hot water overnight with 1/4 cup baking soda. Then a cold rinse (no soap). Then a hot wash with Biokleen or Charlies Soap (in this case, maybe the Lestoil, but I don't use that on the diapers) and cold rinse, with vinegar (1/4 cup) added to the rinse cycle. Then another cold rinse.... Maybe something that might help.