I have a handful of these in my home and really appreciate the small form factor, long battery life, and options for accessing the data (on-screen live values via eInk display, encoded recent history displayed via QR code, or simple http server endpoint over wifi): https://github.com/davidkreidler/OpenCO2_Sensor. It was straightforward to integrate the accessories into my homebridge instance for ongoing monitoring and notifications as well.

You can make one yourself from the information in the repository or buy one from the author at their Tindie storefront: https://www.tindie.com/products/davidkreidler/open-co2-senso....