Case for the DIY-particulate matter sensor from
https://luftdaten.info/
Air enters through a tube on the side (8mm outer diameter) and leaves on the top through a grid. If you install it outside, mount it upside-down such that the temperature sensor and grid are protected from rain. For assembly, you need 11 screws (8mm, outer thread diameter approx. 2.1mm)

You can print the grid using the infill of your slicer!

Print instructions

To print the grid on top, add a cylindric mesh modifier e.g. in Prusa Slicer. Place it in the center of the top part and set top and bottom layers to zero. Use e.g. a triangular infill pattern with 50% density. (Compare to the attached .3mf file)