Try my non-patented budget sound hole humidifier idea, below
Take a Ziploc bag, preferably sandwich size. Poke about 1000 holes in it with a fork. Take a new clean cellulose sponge, soak it thoroughly and then blot it almost dry with a towel.
You don't want it to leak, so make sure it is only moist, not wet! NO DRIPS WANTED
Slip it between the strings, with the zipper hanging out.
Bingo, you're in business. Nothing to leak, nothing to scratch your instrument, should stay moist for a week or so at least, even in the driest parts of the country as long as it's properly cased.
And, perhaps, best of all, if you forget it at a gig you're only out about $.50!
I've been using these for over 20 years and never had a problem. By the way, as far as I know it's my invention!
Cheers
Paul