PolygonHolePunchMenuItem.cs

using UnityEngine;
using UnityEditor;

static public class PolygonHolePunchMenuItem
{
    [MenuItem("Tools/PolygonCollider2D Punch Hole")]
    static public void PolygonHolePunchMenuItem_Click()
    {
        if (!Selection.activeGameObject) return;
        var collider = Selection.activeGameObject.GetComponent<PolygonCollider2D>();
        if (!collider) return;

        var originalPoints = collider.points;
        var newPoints = originalPoints.Clone() as Vector2[];

        for (int i = 0; i < newPoints.Length; ++i) {
            newPoints[i].x *= 0.5f;
            newPoints[i].y *= 0.5f;
        }

        collider.pathCount = 2;
        collider.SetPath(0, originalPoints);
        collider.SetPath(1, newPoints);
    }
}