Small filter adapter. This adapter can optionally take a 46mm fan.

The filter itself requires something like a filter gauze, e.g. from a pack of vacuum bags.

This filter is meant to keep all kind of critters on the outside of your fermentation box.

Note: needs a correction. Screw distance in the STL ist 50mm right now. That's not right. OpenSCAD source is included, but not yet tidy.