After adding the plastic sheet over the structure, as expected, both temperature and humidity have gone WAY up:

Compared with before, humidity range has gone from 40%-90% to nearly 98% almost all the time, only dipping slightly on hot days.  The dew point is now exactly the same as temperature, which can spike hotter, over 40ºC on some days.  I have no idea how this will be for the tea, but it certainly looks happy in there, the tender green shoots are not wilting, despite the heat.  One day, i also discovered a toad happily lounging in a square pot, i guess a moist place to hide during the day is perfect: