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Not sure if this is DIY enough for you, but I'm using an MQ131 (about $20-$30 on Amazon, probably cheaper directly from China) in an air quality sensor I'm building. There's an Arduino library for it [1], and from what I can tell so far, it does work, though it's probably not super accurate. It's also somewhat sensitive to environmental factors, though you can correct for some of that with known temperature/humidity.
[1] https://github.com/ostaquet/Arduino-MQ131-driver
NOTE:
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Hence, a higher number means a more popular project.