I've been working on a "stealth" warm air intake design. It brings in warm air from a pipe that comes in the back side of the air box. The other end of the pipe hovers about an inch above the exhaust manifold.
The downside: You have to make a 2" hole in the back side of your air box.
The upside: It's 1 piece, it's almost undetectable, it's reversible, the engine compartment remains completely stock looking, and it can be installed and uninstalled in less than 3 minutes with no tools, tape, etc.
I'll have a write-up ready with pics later in the week.
As far as blocking your grill goes...
Unless you live where it's really cold right now, I think it's too warm to completely block your grill. I had mine about 90% blocked when temps were in the mid 40's and had to expose more of my radiator because my car would consistently hit 220 on extended interstate grades (even very slight grades). That's too hot for my taste.
I am using lengths of 2" pipe insulation to block my grill. This stuff works great and it's cheap. It almost blends in completely with the grill. The other nice thing about this stuff is you don't need anything to hold it in place. Just wedge it in the rows of the grill. And if your car gets too hot, just pull over and pull off a strip of it. Until temps are regularly in the 30's or below, this is how much I am blocking my grill right now...
It's not pictured...but I have the complete upper opening in the grill blocked as well.
View my fuel log 2015 Mirage ES 1.2 manual: 51.3 mpg (US) ... 21.8 km/L ... 4.6 L/100 km ... 61.7 mpg (Imp)