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 round disk fits over the 60mm hole in back where you would have attached something. We use them in the production facility to test what happens when you keep in all of the available heat.