The drawing in the frame that is rotated left 90°? I confuse easily.
Let's try this: Is the radiator centered or offset left or right? The inlet duct should be longer than the height of the radiator and the opening should be 1/6th the area. A centered duct into a centered radiator is a very direct path. If the radiator is offset to the left (for instance) and the inlet is on the right, then a longer path is available. Then all you need is to make the duct evenly divergent ...maybe turning vanes in the corners.