minecraft_lzstools/data/lzstools_trowel/predicate/mainhand/iron.json

{
  "condition": "minecraft:entity_properties",
  "entity": "this",
  "predicate": {
    "equipment": {
      "mainhand": {
        "predicates": {
          "minecraft:custom_data": {
            "lzstools": [
              "trowel_iron"
            ]
          }
        }
      }
    }
  }
}