The grandparent ships out the SenseAir S8 for CO2 so presumably they're still happy with it. I have no complaints myself but I haven't compared it to any reference standard. He's the founder behind AirGradient. https://www.airgradient.com/open-airgradient/ They have a DIY version that's an open source design and you can either source components yourself or just skip trying to track down everything on the BOM and buy a kit from them that comes with a sleek injection molded housing as well.