modular_storage.json

{
  "forge_marker": 1,
  "defaults": {
    "model": "forge:multi-layer",
    "custom": {
      "base": "rftools:modular_storage_aux#main",
      "Solid": "rftools:modular_storage_aux#main",
      "Cutout": "rftools:modular_storage_aux#overlay"
    },
    "transform": "forge:default-block"
  },
  "variants": {
    "inventory": [{}],
    "facing": {
      "north": {},
      "south": {"y": 180},
      "west": {"y": 270},
      "east": {"y": 90}
    }
  }
}

modular_storage_aux.json

{
  "forge_marker": 1,
  "variants": {
    "main": [{
      "model": "rftools:rftoolsblock",
      "textures": { "front": "rftools:blocks/machineModularStorage" }
    }],
    "overlay": [{
      "model": "rftools:overlay",
      "textures": { "overlay": "frtools:something" }
    }]
  }
}