Karabiner Elements - Double Left Shift to Caps Lock modification. https://karabiner-elements.pqrs.org/ , https://github.com/pqrs-org/KE-complex_modifications

double_left_shift_to_caps_lock.json

{
  "title": "Double left_shift to become caps_lock",
  "rules": [
    {
      "description": "Double left_shift to become caps_lock.",
      "manipulators": [
        {
          "conditions": [
            {
              "name": "left_shift pressed",
              "type": "variable_if",
              "value": 1
            }
          ],
          "from": {
            "key_code": "left_shift",
            "modifiers": {
              "optional": [
                "any"
              ]
            }
          },
          "to": [
            {
              "key_code": "caps_lock"
            }
          ],
          "type": "basic"
        },
        {
          "from": {
            "key_code": "left_shift",
            "modifiers": {
              "optional": [
                "any"
              ]
            }
          },
          "to": [
            {
              "set_variable": {
                "name": "left_shift pressed",
                "value": 1
              }
            },
            {
              "key_code": "left_shift"
            }
          ],
          "to_delayed_action": {
            "to_if_canceled": [
              {
                "set_variable": {
                  "name": "left_shift pressed",
                  "value": 0
                }
              }
            ],
            "to_if_invoked": [
              {
                "set_variable": {
                  "name": "left_shift pressed",
                  "value": 0
                }
              }
            ]
          },
          "type": "basic"
        }
      ]
    }
  ]
}