My Bb boxwood has four keys. Again, when the humidity goes down some of the keys can get a bit sluggish. The Tupperware solution keeps that problem at bay too.
I’ve never had any mildew problems. I keep the humidity between 50 and 60 percent. If it starts creeping up above 65 percent I open the lid until the humidity drops back down.
I actually only rarely have to put a wrung-out sponge in the Tupperware container…I play my flutes frequently (ideally once a day, though life intervenes 
) and usually just keeping the swab-rag in the flute case is enough to keep the humidity level in the Tupperware where I want it. The wrung-out sponge goes in there during the depths of winter, but usually only for a day or two. The key is to get a good tight-sealing Tupperware or Rubbermaid container.