I have a old cooker hood over my cooker as part of the work units - the
type that when you open the 'door' it can start either the light, or
fan, or both. It is linked to a vent in an outside wall.

There is also an opening on the top of the work units over the fan, and
this causes the ceiling to get covered in grease deposits (which then
attract dust) and is a pain to periodically.

Does anyone know if I can safely cover the hole on top of the work
unit?

thanks