Just plugged mine ( LHD US version ). "Nylon Hole Plugs 7/8 inch" ( in case the picture goes way eventually ).
When I got the plugs at the hardware store last night I ran right out and tried to install this intricate mod, but it was dark and I failed to find it.
Ohnoez! There's this huge HVAC plenum right about where it's supposed to go. I tried feeling around back up there but couldn't find where it was.
Took a break at lunch today, dug around a little more and discovered what I think is the proper location in a bit more accessible place, just below and right of the lower center plenum bolt.
It's obscured by the top edge of the carpet and sound blanket.
The plug fits in the indicated hole, so I think I've got it for the time being. No test drive yet.