This is a simple circular cover that I designed for covering over an old light fitting hole left in the ceiling.
It will cover and allow air to flow and so would be useful for old fan vents in walls or anything circular you want to hide.. two screw holes make it quite secure.

it is approx 5 inches in diameter but it scales nicely in slicer software (thats about 12cm) I designed it to be quite robust and to be able to withstand some abuse.