I don't know if I would be comfortable blocking off the radiator with pieces of cardboard right up against it like that. If the fan did come on, it wouldn't be able to draw air through, then engine overheating could be an issue. No aero benefit either. You could always use some tape and mock up a 2/3 - 3/4 lower grill block and remove all the cardboard directly in front of the grill, this would allow the air to be drawn up from between the gaps between the bumper and the radiator...resulting in aero and faster warm up, much smaller chance of overheat with the upcoming warmer temps...and you would have a template for when the color of chloroplast comes along that your like
It would be good to find out how much block you can do safely, then work up to a movable type if you wanted to.
Wheel spats and mudflap removal is another area that helps.