Hi Junimrox
Welcome to ecomodder!
I'm concerned that contact paper would leave a residue, and/or become baked on in the sun.
I've used "painter's tape" for temporary mods. It's a masking tape that has less adhesive that won't leave a residue. It's usually blue or green in color. It comes in various widths like regular masking tape. Even then, I would suggest replacing it every month or so, as I can't promise that sunlight won't cause it to adhere too much to the finish of your new car. You could use it to attach some discs of thin plexiglass over your fogs.
Alternatively, you could use a couple dabs of silicone sealant to hold the plexi on. Just make sure the silicone will peel off without damage. Like the chemical cleaner labels say; "test in an inconspicuous spot for color-fastness".
I bet you could block that upper grille without causing any cooling problems. But I would suggest getting a Scangauge first, so you can accurately monitor coolant temps.