Still overheating

Here is the factory fan shroud for our Jensen radiator about 26" wide core. It tapers towards the center for more volume of space in the center. In your case just put a lip on it 1" tall around the perimeter as mentioned.

Secondly, drill four 3" or 2" holes opposite each fan and put a flap over each set of holes. Hold the flap at the top with flat bar aluminum or steel with screws thru it.

You can see where the flaps go on this factory rad shroud. They have a built in horizontal bar inside the flap hole to help prevent the flap from being sucked thru inwards when the fans are on and there is negative pressure inside the plenum. They also have a horizontal flat bar that goes across the top of the flap and is screwed to the shroud frame to hold the flaps.