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You may have lost the lower shield, as I think they had shields on the top and bottom. Use a large screw-type hose clamp, or two connected head to tail, to hold the shield in place. Cheap fix, and will last the life of the truck.
FWIW, Ford will search high and low to save 10 cents on each unit they produce. If they can cut out a part, they will. They would not put the shield on the cat unless they felt it was necessary. The shield also will prevent the infra-red of heat from cooking the bottom of the cab - your feet, or rather passenger feet. And carpet and padding. We have a screen in front of our fireplace to prevent sparks from escaping and starting the rug on fire. You can feel the heat output drop when you put the screen in place. The rays will not pass through the aperture in the screen. Same thing on the cat shields. They do prevent touching a hot surface but ALSO prevent the heat from passing.
tom