Baking soda sprinkled liberally in the storage container with the problem and left to sit for an extended period can help greatly.
I had a cell phone in a plastic case that took a nose dive into a portacan and was retrieved by the pump out guy the following week.
Just as an experiment I placed the case in a zip sandwich bag with a bunch of baking soda. Initially I didn't think it was doing any good, but came across the bag with the case and baking soda a few months later and there was no odor remaining.
So not a quick fix, but worth a try.