I finally got my terrarium set up for lowland nepenthes and a couple of future drosera petiolaris sundews. With a humidifier with a tube to use as needed and loosely placed bubble wrap over top (not fully sealed), I can both trap heat and humidity and it's working. (humidity is in the 80+% and temperature gets to about 30 degrees Celsius). The base consists of lava rocks, water, and a carpet of dried sphagnum moss.
However, I'm not sure if I should be concerned about air flow. I bought a small computer fan and adapter so it can plug in. Previously I had tried using it before the bubble wrap idea, when I just had an aluminium foil tent over top. The fan both caused loss of heat and humidity back then. With bubble wrap (and maybe an added foil tent over top if necessary), maybe this isn't a concern. Should I be using my fan? If so, how often? During the day and night, or only during the day, or night?