Loner Life in Another World 8

OMUI BEHIND THE EIGHT BALL? NO WAY, NO HOW!
With the invasion from Lord Narogy halted, Haruka sets his sights on a bold new goal... A fake labyrinth! A dud of a dungeon! A bona-fide faux-cave! If he can make it so the only way in or out of the frontier is blocked off by a deadly cavern, then our reluctant hero knows Omui will be safe until it's in a better position to defend itself! But to create this decoy deathtrap, he's gonna need to line it with the most ridiculous deterrents he can think of. And for once, just once, this lackadaisical loner decides he could use a little help from his friends!
What will they cook up? How will they test it? And just who else might be setting envious eyes upon the frontier's bounty?