This is a fan vent cover for our Prusa i3 MK3S acrylic enclosure..
There are two goals in enclosing a 3D printer: keep in the heat, and improve indoor air quality. Some people just care about keeping in the heat, and for them they can ditch the whole fan/vent/filter options and just cover all the holes.
This fits over the 60mm hole in back where you would have attached something.
Its just a round disk with holes in the right places.