A marine deckplate, hence the term "Deckplate Mod". The cap screws back in when needed and is gasketed so you can off-road and forge creeks if needed. Open, the airbox will NOT suck in any rain water. I've driven thru some terrible downpours at 70-80 MPH and never got a drop of water in the airbox.
I added a piece of window screen material held on by a large hose clamp on the inside flange of the deckplate inside the airbox. This will help prevent the larger "chunkies" (bugs, leaves, pebbles) from entering the airbox without sacrificing airflow.