When Ishitaka found an unsealed bag, I definitely thought "oh, Kaiji filled it with porn mags as a final trap"
But from Ishitaka's response, I have to wonder... is it something embarrassing... that *isn't* porn mags...?
I guess gay stuff is the most likely. He's going to pretend that they're a gay threesome, maybe? FKMT has been making jokes about gay stuff recently, so it'd just feel a bit... overdone in this series (if people recall the Teiai camping trip misunderstanding)