I am in the planning stages of moving a cold air return from an inconvenient spot in the floor to a nearby wall. The basic plan is to open up the wall on both sides (cutting one stud and framing in a filter grille box) and cut a smaller hole in the floor on the opposite side of the wall from the filter – the "back" of the wall.
To make this work, the incoming air will need to make a 180º turn from where it passes through the filter to where it enters the main return flex duct in the basement (16", if that matters). We don't have a lot of room on the back side of the wall, so need a compact solution – <14" outside the back side of the wall – that won't create excessive noise. The system specifies 850 cfm.
What parts/products should I use to route the air like that? I know there are companies that create custom returns, which I'm open to if the price is reasonable, but is there an off-the-shelf solution? Is this just a bad idea to start with?
-- Edit --
Here's a sketch: