This "大空洞" (Daikuudou) is the last dungeon in the game, the "Northern Cave". I know the best translation for it is "Great Cavity", but where do I insert it in that sentence?
The "best translation" is not
"Great Cavity". The first definition I see for 空洞 is "cave". I would honestly just leave it as "Northern Cave", since it sounds better than "Great Cave". You could also call it the "Great Cavern". Remember: when people go spelunking, they often find themselves going down vertical caverns such at the one we're discussing. They're caves. Not "cavities" or "hollows".
or, less engrishy... "Shut up! I don't wanna hear your blabber! Guests like you should just sit your asses quietly down in those chairs!"
I was purposely being literal so he could see what the original line said. Whereas you're adding random words (including curse words) that weren't actually in the original line.