Smelling a pear. This will be running for a week+ probably since pears are sold so under-ripe.
These gas sensors are just crazy sensitive to any airflows! With the previous setup, I would notice sensor values slowly begin to take off whenever I sat down at my computer next to the chamber and stay there.
Since these sensors are measuring the gas by depleting it, those elevated sensor readings meant some external gas was continuously entering the chamber somehow.
So my goal now is to eliminate as many holes as possible in to the chamber. I got an Adafruit inductive charger kit so that I can wirelessly power the fan inside of the chamber, and I also whipped up a small adapter PCB to route all of the sensor connections in to one cable. This way instead of having to cut 8 holes in to the chamber lid for the sensors, I can instead cut one hole and mount an o-ring sealed cord-grip in it:
This particular cable is definitely overkill for this situation, but it was the only one I could find that I could buy by the foot instead of by the spool. McMaster FTW (as usual).