Problem part of a network surface

 From:  davidb45
I am trying to create a car front bumper with an air scoop cutout. Creating the network surface is easy enough using either the Sweep or Network tools.
In my particular situation trying to cut out the scoop opening is problematic. I have tried Trim, and Extrude/Boolean Subtract. Can anyone tell me what I'm doing wrong?

.3dm file attached.

Thanks in advance.