Luigi only needs to enter the fishing hut once. In mission D-1: Cold Case, he must find a Toad painting inside the hut. To do this, shine the Dark-Light Device on the back wall and point it up. Luigi can return to the Fishing Hut in mission D-3: Across the Chasm to find a Gem. Use the Poltergust 5000 on the snowmobile in the left corner to get the Gem.