Your in a relatively dry climate then, I get that much humidity with one of the small water holders. Well, in late winter it takes both the small holders. Any who, there is plenty of space from the three holders to edge to lay in a sponge or two. Humidity is relative to surface area of water in unit so a sponge provides plenty of area.
If you have not calibrated your hygrometer I suggest you do now. Put in ziplock bag with bottle cap full of a salt paste (just enough water to make a paste not slurry). After 6 hours it will be 75% humidity in that bag, adjust your hygrometer acordingly.