I have just finished PCB layout for the Air Quality Mini Station. I decided that I would put CR2032 backup battery holder on the back to save some space. All other components are on the TOP layer. Most of the board is for expansion PCBs - both for MCU and additional sensors.
Overall dimensions of the PCB is 100 mm x 50 mm, ideal to fit two of these in the 100 mm x 100 mm standard PCB in SeeedStudio Fusion service. I have ordered a first batch of 20 boards (2 x 10) for ~$5 + shipping. As soon as I get my hands on them … soldering time! 
