Geometric flush mount cold air returns
These flush mount pieces are easiest to install when building new construction because then they can easily be built into the wall. Retrofitting can be difficult. Take a look at the pdf install guide and you'll see what the story is. Still, when you get done you have a great clean looking installation.
The grilles are held in place by very strong rare earth magnets that take a lot of pulling to dislodge. Feel comfortable in installing these in a ceiling. They definitely will not be falling on your head.