The supply and return are split in the middle where the screws are.The return starts on the far left side of the duct.
Here is what it looks like inside the unit. The clear opening dimension is 7"x18". This is a 5 ton unit. There is a 20x30 filter grill in the house.
notice the rust from condensate sitting. The condensate only drains when the unit shuts down.I didnt have my magnahelic this day but you can bet that the static was well over 1".