DIY CO2 monitor?

Hello,

I'd like to assemble a CO2 monitor with the following features:

• < $50 (Aranet is nice, but 200 is a bit over budget)
• NDIR sensor, as they're more reliable than COV
• LCD display
• ready-to-use box, not 3D-printed
• Handheld so I can take it with me (powered by either a regular, rechargeable battery, or a small USB powerbank)

As there are different sensors and boards available and I know nothing about that field... if you've already built one, what would you recommend?

Thank you.
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