This system allows you to create custom menus with item slots and 3D entity renders. Files must be placed in:

data/corewithstuff/cws_menu/jsonname.json

{
  "background": "minecraft:textures/gui/container/furnace.png",
  "width": 176,
  "height": 166,
  "type": "custom",
  "store_items": true,
  "slots": {
    "slot_0_fuel": {
      "x": 56,
      "y": 53,
      "limit_items": true,
      "items": [
        "minecraft:blaze_powder",
        "minecraft:magma_cream",
        "minecraft:ghast_tear"
      ]
    },
    "slot_1_input": {
      "x": 56,
      "y": 17,
      "limit_items": false,
      "items": []
    },
    "slot_2_output": {
      "x": 116,
      "y": 35,
      "limit_items": false,
      "items": []
    }
  },
  "render_entity": true,
  "entity_settings": {
    "type": "blaze",
    "size": 25,
    "position": [25, 45],
    "mouse_follow": true
  }
}