FWIW, I put my humidity devices in the accessory box, which is NOT airtight. The moisture propagates through the whole case just fine over time. Don't believe me? Put a raw onion or a clove of garlic in there and see how your case smells a day later.... There is no risk of leakage directly into or onto my instruments. For my Taylor guitars with cutaways, I sometimes put a cylindrical humidifier (sponge in a Crystal Lite drink container wrapped in my strap or a cloth) into that void as a supplement when it is really dry, but not always.