Is that a photo of your car?
I've just had another look and feel, and as much as I could see under drivers seat and its not like that. I even raised passenger seat to most forward position.
Both seats look very similar, with a big cable. There is an area which looks similar, but if I slide my hand in, there is no opening other than what feels to be a narrow slit if I clench my fingers right back. Now I may have mis interpreted that photo, but I was expecting a large opening with a vertical opening - if its like that, mine has a cover over it already. I could imagine something getting down the slit, but to get there, its into the opening in the carpet and back under yourself - infact, what I may be feeling may even be the gap between the carpet and the floor. There definitely is something there, but not some huge void into the top of the battery or anything resembling the sort of view in those photos.
I wonder if the design has changed or, our car had a SC visit where some of the inside sounds as if it had been removed and I wonder if it was modified in some way?
I can't honestly see any way to fit a grill in that area. It would be like sliding a vent cover into a narrow opening in the carpet.