Several clutches of mine tend to mold over if I cover the Petri dish. I have a plastic storage bin with a 'false bottom' with a few inches of water. The Petri dishes sit in a deli cup and the cups rest on egg crate that rests on stands. The deli cups just barely touch the water. (1/2")
There is a small aquarium heater in the water to maintain temps.
You still need to mist as needed (sometimes daily)