It's rare nowadays to need a replacement air filter box housing. Sure, on older engines they were made of metal and had the tendency to rust over the years. Also, the older the car is then the more likely it could simply be missing. A lot more work was done on carbureted engines in the past that is done on today's fuel-injected engines. Modifications were easy, and it's extremely likely that an old Camaro or Mustang with a small block V8 doesn't have the original air filter box. But that doesn't mean that any substitute will suffice when one is needed. A properly sized housing is absolutely necessary in order to fit under the hood, to line up with the intake hose boot, and carry a filter element that will sufficiently clean the air before entering the intake manifold.
The air filter box on most modern vehicles is made from a stiff ABS plastic, and is molded to install in one of the corners or in front of the engine compartment. Typically it consists of two shells fastened together by a couple of stiff clips. Often times it will be mounted to a bracket on the fender or radiator shroud, and is located in a place that will most easily draw in a constant stream of air while driving. The further away from the engine it is located, the cooler it remains. And cool air is key to maintaining fuel efficiency and power output. Additionally, as the air is brought into the filter box, it is tumbled and resonated. This means at low speeds or at start up it can draw in more air than normal, and at high speeds or when the engine is warm it can draw in a little less.
Installing your air filter box is really easy if you have an old carbureted engine, and a little more challenging if you have a newer fuel-injected engine.