I never fitted that shelf or put any holes into the grill block and may not as Helga is due to be replaced end of June when Mrs A's Prius arrives. The way she is sipping Diesel at the moment that may not even leave enough time for a full tank to see any effect
Mrs A's Octavia (which I will probably take over) is similar behind the grill - the whole space is open behind the bumper. SuperSkoda do a block for the Octavia but I have been told a legend of an OEM block though which may be cheaper.
EDIT - PS, I like the walling off - may look at that too.