Craig, this is how I did mine
I wanted some airflow to still get to the valance grille
You can just see the stainless 'drop brackets' on this close up, the Perspex plate is backed up with a sheet of Ali and the brackets are very slightly formed with an outward twist so that when tightening the bolts up it stresses the plate to form a bow that shadows the lower grill curve
Close up here..
