You put it in a shallow cup/pan of water level about 1-2 inches higher than the unit, and it will continuously spit out tiny droplets of water into the air. The one I posted the link for is normally used in ponds for the "mist" effect. This one works very well if you set it up with a humidity controller, or you could set it on a timer to turn on then off so many times per day if you figure out what timing gets you to the correct humidity.
Another idea (if you don't want to buy the above) would be to get a wide pan of water rather than a bowl. The larger the surface area of water to air, the more evaporation. Adding a paper towel/cloth folded so that it sticks out of the water (as a wicking agent) helps as well. Just make sure the paper towel/cloth isn't sticking outside of your container of water, or it'll wick all of the water right out! This is also harder to do when you don't have the space, like me. I've only got my small mini fridge as my cave
And lastly, if your cave is hitting 75% regularly you can always use minicaves! From my experience, if your cave is 70-80%, your minicave will be 90-95% depending on the moisture content of the cheese inside it. You can always double up with water bowls and put one inside the minicave as well to up its humidity!
Hope this info helps!