minecraft-assets/data/minecraft/recipes/shulker_box.json

{
  "type": "crafting_shaped",
  "pattern": [
    "-",
    "#",
    "-"
  ],
  "key": {
    "#": {
      "item": "minecraft:chest"
    },
    "-": {
      "item": "minecraft:shulker_shell"
    }
  },
  "result": {
    "item": "minecraft:shulker_box"
  }
}