Just to avoid confusion: The original IKEA board now only powers the sensor. No exchange of data whatsoever.
POC
Final Board
Final product
Not the prettiest board I could imagine, but it gets the job done. The only downside is that the LEDs don‘t mean anything anymore, but putting more wires in there to connect them to the ESP to actually mean something was too much of a hassle for me. Planning on communicating critical values anyway via Alexa.
Sorry, I don’t remember precisely. But I considered two conditions: Putting the sensor‘s inflow close to the grill of the housing. And placing the sensor in the allowed orientation in the specs.