This ventilation plug insert fits into a smooth 2.5 inch diameter hole in a material having a thickness of approximately 0.25 inches. The small ventillation holes are spread out over the surface of a spherical dome. Holes in a larger curved surface area results in improved ventilation as compared to holes in a flat vent. Originally designed for an aquarium application to separate fish into different compartments but can also be used as a ventillation screen in air. The plug is held in place in the hole by the "V" shaped flange on the bottom. Just pinch the tabs when inserting the plug into the hole until it pops into place.
Such a grommet can be used in various projects for air exchange or interesting visual effects. In electronics equipment it can be used to allow hot air to escape the top of an enclosed chasis or to allow internal lights to shine out of the enclosure. In reptile or insect cages it can be used to assist in air circulation and exchange. In an aquarium it can be used to allow water circulation and keep fish where they belong. Since glue or other fasteners are not needed to keep the plug in place it is more easily removed for cleaning or replacement.
One last thing. If you are a fan of triangular shaped holes instead of hexagonal holes such a version may be found at the following
link:
https://www.shapeways.com/product/3BVHWZA6C/vent-insert-2p5-inch-hole-flexy-plug-dome?key=0e1740e9dd26ca2821a4eedb8014e14d