Just for who might be interested.
I was already looking for a long time for a good CO2 meter but either they were too expensive or too unreliable. So finally I decided to create one myself with the help of Homeyduino. The sensor itself is from Senseair (S8, auto calibrating), a reliable company when it concerns these type of sensors. The sensor is connected (via UART) to a TTGO ESP32 with display so the CO2 value can also be read directly. When the CO2 value exceeds 1000 parts per million I will be notified by my Google Home.
I must admit the enclosure looks horrible but by the lack of some good tools and patience it became what it is. At least it works perfectly well.