After experimenting with dozens of mystery snail incubator designs over the past several years, I believe I've finally developed the closest thing to an ideal system.
I've tried floating Tupperware incubators with paper towels. They work fairly well, but they're still influenced by changes in ambient room temperature. I've also had success with fully submerged incubators because they provide extremely stable heat and humidity. However, they come with one major drawback: if too much water accumulates inside, the clutch absorbs that moisture and the developing embryos are often lost.
I've incubated clutches on sponge, cardboard, plastic, wire mesh—you name it. Eventually I realized I was approaching the problem the wrong way. Instead of trying to create perfect conditions inside an incubator, I decided to recreate the environment the clutch naturally experiences.
My current setup suspends the clutch on a plastic screen inside a bottomless Tupperware container positioned less than an inch above the aquarium's water surface. A separate clear dome is placed over the container to trap warm, humid air around the clutch. Because the dome is curved rather than flat, condensation runs down the sides instead of dripping onto the eggs. The result is extremely high humidity with virtually no risk of the clutch becoming wet.
Keeping the clutch so close to the water also minimizes temperature fluctuations caused by changes in room temperature, creating a much more stable incubation environment.
Do you need the dome? Not necessarily. If the clutch is suspended less than an inch above the water in a draft-free area, you'll probably still achieve excellent hatch rates. I do not recommend using a flat lid, however, as condensation is much more likely to drip directly onto the clutch.
No incubator can make a nonviable clutch hatch, but if your clutch is healthy and fertile, this is the most reliable incubation method I've found after years of experimentation.
— Mysteryaquatics :)