NamespaceFillerAssetProcessor : Fills in #NAMESPACE# variables in Unity script templates. Source: https://stackoverflow.com/a/52395369/6301627

NamespaceFillerAssetProcessor.cs

using System;
using UnityEditor;
using UnityEngine;

public class NamespaceFillerAssetProcessor : UnityEditor.AssetModificationProcessor
{
    public static void OnWillCreateAsset(string path)
    {
        path = path.Replace(".meta", "");
        var index = path.LastIndexOf(".", StringComparison.Ordinal);
        if (index < 0)
        {
            return;
        }

        var file = path.Substring(index);
        if (file != ".cs" && file != ".js" && file != ".boo")
        {
            return;
        }

        index = Application.dataPath.LastIndexOf("Assets", StringComparison.Ordinal);
        path = Application.dataPath.Substring(0, index) + path;
        file = System.IO.File.ReadAllText(path);

        var lastPart = path.Substring(path.IndexOf("Assets", StringComparison.Ordinal));
        var _namespace = lastPart.Substring(0, lastPart.LastIndexOf('/'));
        _namespace = _namespace.Replace('/', '.');
        file = file.Replace("#NAMESPACE#", _namespace);

        System.IO.File.WriteAllText(path, file);
        AssetDatabase.Refresh();
    }
}