If you add a diffuser in the back, you will most certainly need an underbody for it to be effective. If you can make the underbody curve upward slightly into the diffuser, do it.
Cold air intakes do kinda bring colder air in, but as long as you're in the engine compartment, it's not going to be that effective. If you really want cold air, you form fit an opening from the front of the car to the filter, and have everything sealed off and wrapped in some sort of thermal insulator. Then you'll really get some cold air
This will also put a positive pressure differential on the intake system at higher speeds, like a turbo does, but not nearly as much psi.
Congrats on your hypermiling, and thank you for your service!