Forgive me brothers and sisters for
I have sinned against this very principle...
like this! (aero fog light cover)
Albeit that they are yet far away from the edge (at the right of the picture) so it probably does not disrupt the effect too much.
It may even be good, there is a sharp ridge at the edge of the bumper anyway.
And it does solve my main concern: protecting the fog lights from stone strikes.
The covers are curved though the photo does not show that. The curve makes the thin plastic stronger.
Then I cannot say it helped FE wise, it got worse last 2 weeks.
But I had to air up the tires a bit again and one of the brake disks is slightly warmer than it should be despite the extra pad return spring.
Too many variables, too small an effect.
__________________
2011 Honda Insight + HID, LEDs, tiny PV panel, extra brake pad return springs, neutral wheel alignment, 44/42 PSI (air), PHEV light (inop), tightened wheel nut.
lifetime FE over 0.2 Gigameter or 0.13 Megamile.
For confirmation go to people just like you.
For education go to people unlike yourself.