Moved the sensor again, the lid temp was too warm to be useful. Now it's coming across the hanging basket, right next to the dial thermometer. Which I now know reads 5 degrees C or so cooler than the sensor in the same location. (But the sensor wire might be conducting warmth from outside or something, so I don't know for sure which reading is more accurate.. Will add a second analog thermometer to calibrate..)
Interestingly, you can now see every time I open the fridge door, because the temp rises half a degree or so, and then slowly drops back down, but never quite to as low as before. Each door opening costs maybe 0.2 degrees in banked cold.
(I was just comparing sensor readings, not snacking all evening, honestly! )
Otherwise behavior last night was more or less as expected, it was still cooling off the air until 3 am and then crept up slowly.