Agreed, the difference between "real" CO2 meters and consumer CO2 meters is really stark. Given that you can get nitrogen and CO2 in tanks, I expect one could build a calibration setup with a fixed volume tank, two metered gas ports, and some software. Start by purging the tank with nitrogen, then add CO2 for the 400ppm baseline. Calibrate there, then add increments of 100ppm (or 50ppm) CO2 and record the readings up to say 1500ppm total. Then you have a calibration table for that meter that will let you translate from its indicated ppm to the real ppm.