An air vent is rarely the place burglars look for valuable stuff in. However, it's also a horrible place to use as a safe, regardless of what Walter White will tell you. The Air Vent Secret Compartment offers a compromise, giving you a proper wall safe that looks like a regular air vent.
Made by Hidden Safes, it's, basically, a small cabinet that you can install in any wall with studs that measure 16 inches or greater from center. Cover it up with the included drawbridge and it looks just like a standard part of your home's air distribution system, lessening the likelihood that burglars will even bother checking it out.
Don't worry, the Air Vent Secret Compartment doesn't open like a regular air vent. Instead, the cover actually features a fastener hidden in the louvers. Turn that a quarter of the way to disengage the locks and automatically open the drawbridge. The interior can be used as a single wide and tall (but narrow) space (for holding a briefcase) or arranged into compartments using a pair of included shelves. It measures 14.5 x 8.25 x 3.75 inches inside and 15.5 x 9.5 inches outside.
According to the website, installation should take no more than a few minutes. Just cut a hole in the drywall or paneling (yes, you can use your unbreakable hatchet for that), put four screws in, and you're done.
You can get the Air Vent Secret Compartment directly from the Hidden Safes website. Price is $259.95.