I used the clear plastic that comes in cheap poster frames at wallmart.
I wanted to still see the custom grille I built last yr.!
there is an aftermarket body kit (IMPUL) that is 2 inches lower than stock. So I put the clear plastic over the custom fog light...
these two items pushed me over 28.5mpg at 62mph. I had gotten 27 prior.
Just one detail: you clearly see the tape, don't you?
You could avoid this by blocking from the inside, just as I did: http://www.gassavers.org/showthread.php?t=11766
My 60% block is inside. A variable part I can only use on shorter distances is outside and slides in/out in really one or two seconds. Than I have 90% blocked.
Advantage of your block is some more aerodynamic performance than blocking from the inside. But the difference will be very very small.
One question: if I'm right you blocked 100% of the upper (main) grill and the underside grills is practically blocked by your license plate => no cooling problems? Especially on highways.
I did not want to do a partial. my goal was to use the thinnest and clearest material. I wanted to maximize the aerodynamics. that's why the block follows the lines of the body.
there is plenty of air coming thru the lower grill.
also, I have a custom lower air duct installed in the driverside lower opening