Hi doomthings, the reason we say to "cover loosely" 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 lid to pop off. On the other hand, we've found covering the starter with cheesecloth or a towel allows too much airflow, and the starter surface tends to become crusty, which can restrict rising. As long as you leave sufficient headroom above the starter level for the gases to accumulate, and you open the jar now and then to vent the gases, it's fine to use a lid. Covering the container with plastic wrap is also a good solution, since plastic wrap will flex. 

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