It probably doesn't even need a board, you can just wire all the sensors in a box and solder the cables. Each sensor only has 3 or 4 and so does the display.

But a board is cleaner and they're super cheap to get made in China unless you're shipping to the US I guess lol.

I've built sensor boards like these too. But more with Dallas one wire sensors to measure temperature in different locations of central heating systems.

For sure. Was just curious as they started out with a super basic DIY setup https://www.airgradient.com/documentation/diy/ (which can be implemented without the PCB even, as another commenter in this thread suggested) so I was wondering if there was a functionality you found was missing that sparked you to create this project!