I just use these:
http://www.walgreens.com/store/c/wal...188636-product
I unfold the pad slightly from the plastic wrap, dampen with water (these are super absorbent so they hold quite a bit of water), squeeze out excess if any, close up the plastic wrap and stick it in the case. I get 45% humidity right on the dot.
I use a radio frequency thermometer/hygrometer in the case like the ones from
http://www.amazon.com/Ambient-Weathe.../dp/B00FX8ZGOO