Season your box with an 84% boveda pack. Put it in, close the lid, don't touch it for a week.
No fuss no muss.

Then pick your preferred humidity (they come in 69, 72, 75, 84, percents) and switch it out and you're all set.

Those sponges are so much more of a hassle, a boveda will work for 2 months without the need to change em and they're crazy cheap to replace.