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 [1]) in an air quality monitor I'm building. There's an Arduino library for it [2], 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, but you can correct for some of that with known temperature/humidity.
[1] There are a few different kinds of MQ131 (for high and low concentrations); you'll probably want the low concentration one. I also removed the bare sensor from a breakout board I purchased so I could access it directly from my custom PCB.