I see returns like this all the time. Problem number 1.) There is no return air plenum. If you look at the Before photo, you can see its just a 2x6 frame. Problem number 2.) The top of the return frame is AC Ductboard. Whats wrong with that? Take a look at the sides of the duct board - it's fiberglass insulation which does not stop air flow. So when the unit is running, it is pulling air, dirty and unfiltered air, in through the fiberglass (look closely and you can see that it is discolored, dirty) and straight into your home!
Our solution for this was to seal everything at the base and then spray with closed cell foam.