Right Out in the Open
It doesn't have to be an old vacuum cleaner. Any common household item that has a cavity will work. Think old printers, computer towers, children's toys, etc. (Just be sure family members know about it so your valuables don't get donated or tossed!) For easy access, choose an item that opens instantly, like a vacuum cleaner bag compartment. For more security, choose an item with a cover that screws shut.
Cut out a stud space opening to fit a return air grille. Cut off the grille screws and glue just the heads in place. Run four drywall screws into the corners of the opening so they fit just inside the rim of the grille. Then glue rare earth magnets to the back of the grille so they line up with the screw heads.