Got a $60 CO2 detector. There are much cheaper options but this one has a proper NDIR sensor and has good online reviews outside Ali.
Needs Wi-Fi connection and it sends data to the cloud so I'm now (willingly) on the chinese botnet. I could MAC filter it at the router but I don't really care. It works without Wi-Fi but can't be configured.
As for testing, this early morning I closed my window because of some loud bike asshole, woke up at 9 and it was like 1900 ppm (red zone starts at 1400), didn't expect it to rise so high in such a short time, I'll try closing my door and window tonight and see how high it gets.