Not the OP, but I really like the SCD30 by Sensirion. It's the sensor used in the widely popular Aranet4, but the combo of sensor+MCU when you DIY it costs about a third of the commercially integrated product, so the feeling of thrifty hacker accomplishment is a nice bonus on top of the good HW.
The SCD30 also is a dual channel or dual beam NDIR sensor, meaning it doesn't really drift. Unlike most other ones that need the room to have outdoor-level CO2 once a day or week or so to calibrate.
