I'm with Adam. A box of baking soda poured in a try might do the trick.
The humi is veneer. Very, very, thin veneer. Take a piece of sanpaper to it and it's history.
If the baking soda doesn't do it, I'd ramp it up to a 10% bleach solution and spray it down or soak the whole inside in it for a few hours. You could do the base then prop the humi to do the lid.
If that doesn't do it, bury it in the back yard.
