If you design it right, the fan should not have to come on at all (it might in stop and go traffic).
It would be hard to make it be flush to the grill. I do not think that it is needed though, as long as the air is not entering the engine, it will just stack up and form a boundary layer.
If you look closely at my grill, it is indented by about an inch, I could cut out an opening for the back side of the grate to come out of. then when it is closed it will be flush, when open the fins will stick out about an inch. I do not want to cut my grill (not reversible) but I think that the lower gill portion (dark gray in pic) can pop off.
A junk yard near me has 2 matrix's so I might go buy a second one to cut and mess with.