Passenger side water leak - extra part

[monkeylizard]

I had the common problem of water in the passenger floorboard.

Removed the cowl, installed a cabin air filter, and reattached the weatherstip that was almost completely off.

I have an extra part that appeard to just be laying in the vicinity of the blower, though it could have come loose when I removed the cowl. It's black plastic, about 12"x6". It has a slight curve to it and some louvered slots in it. It has 90 degree edges on the two short ends that make me think it's supposed to fit over or inside some edges, like in a box. There is no part number and I can't figure out where it would go. I thought it might be a cover for the cabin air filter area, but I couldn't make it fit. Any idea what this thing is and if I need it? It's currently in my box of extra parts...