Hi folks,
I found a pool product last summer called a "water noodle" that is a foam plastic cylinder the same size as the front bumper inlet. A quick slit on one side and it fits right over the center, horizontal grill of the NHW11 Prius:
This is the part:
http://www.pooltoy.com/waternoodle.html
It is easy to shape with a knife. Designed for swimming pools, it is water proof, soft, and probably UV resistant. For less than $5, it is an affordable solution.
GOOD LUCK!
Bob Wilson