It wasn't designed to be extra inventory. Its functionality appears specifically to prevent it being used as such.
What I do? I get a bunch of them. I put one in the entrance to the mines, one in the entrance to the skull cavern, one right outside the volcano dungeon, one in my house, one in my crafting shed and one in my hut on the island.
Generally, when I'm on the island and have stuff I plan to bring back home but have stuff to do on the island still, I put the stuff I want at home in the junimo chest. If I have stuff at home that I want at the island, same thing. The one in my crafting shed is for taking out stuff that goes in there. The ones at the mines, skull cavern and volcano dungeon are only for getting stuff out of my inventory before I go in.