Vinegar?

You could try crumpled newspaper or paper towels, along with a charcoal briquette or two, and put the lid on for 24-48-hours, repeat if necessary with new paper and briquettes. Paper and charcoal absorb odors fairly well.

The problem with plastic is that it is porous, and any product that comes in contact with it that has an oil (even flavoring oil) or fat in it bonds with plastic rather than just sitting on the surface like it would if it were packaged in glass (which is non-porous), and is nearly impossible to get rid of.

-Grainlady

If you are using plastic quit. I had a very bad experience using plastic cereal containers. I don't use any plastic for food stuff anymore. I use square glass container for storing food.

I repurpose plastic juice containers sometimes, though I don't normally buy commercial lemonade....

I wash the containers in hot soapy water, and rinse, rinse, rinse again. Then I leave them sitting, tops off, to dry and air 2-3 days before capping and storing. I haven't noticed persistent odors.

I had 4 cereal containers and I opened one and the awfullest odor I have experience came out of that container when I opened it. Dry cornflakes cannot possibly cause that odor, it had to have been the plastic. That is when I dumped my plastic. Earlier I had stopped using plastic for storing wet things like soup.

Thanks for the helpful comments!

Going to file away those paper-charcoal odor-removal suggestions for later.

Will try vinegar, soapy water, just to see what happens, but noted that a different plastic (PETE #1) lemonade container did not maintain any odor - which supports the above idea of a particular plastic+flavoring reaction.

Not always easy to avoid plastic - and in this case the intended use certainly wasn't anything critical - but (off-topic) for consumables I especially try to avoid plastic+heat as in a microwave.