[MetroMPG]

Aside from dealing with the sag, I'd try to radius the "shoulders" of your mod. Aerohead alluded to this already in his comments.

The sharp corner transitions you have made (between "top" and "sides") are easy to fabricate, but more likely to "trip" airflow into turbulence (not good) or vortices (bad). Note that the bodywork that Toyota made at the trailing edge of the roof/sides has fairly generous radii.