Hi Lisa, the reason we say to cover the starter lightly isn't because the starter needs airflow in order to ferment properly (it doesn't), but because fermentation gases can build up in a tightly lidded container and cause the top to pop off. As long as you leave some head room above the starter level for gases to accumulate, and open your jar now and then to vent the gases, an unsealed Mason jar lid will work fine. Plastic wrap is also a good option. I would not recommend cheese cloth or a dish towel, as these can cause the surface of the starter to dry out and crust over. 

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