Thank you both.
The lid will stay on almost always. The idea is to create a more humid environment. These plants thrive in very moist conditions, and bringing them into the house from outside means they'll be warmer (nothing I can do about that) but also a lot drier. The drier environment is more perilous to the plants than the higher temperature, so I'll control the evaporation by keeping the lid on. Even at that, it is not a very tight fit, certainly not gas tight. The lid used to have a handle on it since it probably began life as a lid for a saucepan. There's a tiny hole in the center. Also, it is just resting on the glass rim of the jar. It's a little rattly and I know there is room for air to move through.
What air movement there is though is likely to be minimal. This also means it will stay humid and moist, just what these little guys like.