Honestly I am trying to figure out why you would need something that thin. Is this a fake tunnel facing a track and you need clearance? Would you wall it up, because even if you paint the interior black it will reflect some light and be obvious. Or maybe it's a working tunnel and there is something in the way? I don't know if a tunnel portal that thin will ever look convincing.
But that said, what about ripping down a plastic tunnel portal on the table saw, or carefully with a hand saw? Or use thin foam and build it up block by block right on the drywall as others have suggested.
