[aerohead]

Commercial awnings on buildings around here are surviving 60-mph + thunderstorm gusts, fully exposed.
They're just tensioned Hypalon-coated fabric, laced to a very light, heli-arc- welded aluminum skeleton.
'Hiding' in the lee, behind an airdam, a taught fabric belly would be subjected to very little force.
NASA only gained a 5% drag reduction for a full belly pan on their semi-trailer, but hey, it's 5%.
If splash & spray is isolated, and cannot loft debris and water up on top of it, it seems like an okay 'solution.'
' Enclosed' within a perimeter of the airdam, 'rocker panels', and a diffuser, there'd just be dead air above it, and 'parallel' air licking the underside ( inverted bird bath on the belly ).