The “Air Vent Secret Compartment” is just that, slipping into walls with 16-inch or greater stud spacing. A small latch hidden inside the louvers opens the front.
It’s clever, but less so if the vent cover doesn’t match the ones in your home. Also, it’s a shame it doesn’t have a safe or lock integrated inside, but I suppose you could also place a smaller lock box on its shelves. The Vent Compartment is sold by HiddenSafes.com for $275, shipped. (They’ve other types of safes, as well.)