[lunarhighway]

i found some details on the calibra bumper with with numbers about the size of the cutout

of course it did not have such a large dam as your can but it's more detailed than what i've found about other cars

the original article is in german
but i've ran it trough yahoo bablefish wich makes it somewhat readable in english with some goodwill and imagination.

interestingly the research diagrams show that they did a great deal of optimising, increasing the size of the cutout untill they noticed an increase in drag again.

perhaps it's doable to make a large cutout in the center, than tape a flat piece of cardboard on it that you can raise and lower, first in a few large steps to see where drag gets worse and than refine it untill you get the optimal height