I finally got around to implementing this which had been nagging me for some time.
It’s a data logger thrown together from a dime-a-dozen STM32F103 Blue Pill board, and an almost-as-cheap cheap Bosch BME280 module (temperature, barometric pressure, and relative humidity). It logs to the F103’s on-board flash (tons of memory left over after the code is programmed) so no SD card needed.
There’s no hardware to speak of to be open source, though. Maybe I’ll do an enclosure at some point.