Demonstration of the pagetemplater.php plugin - https://github.com/damiencarbery/page-templater/blob/master/pagetemplater.php

pagetemplater-demo.php

<?php
/*
Plugin Name: Page Templater Demo
Plugin URI: http://www.damiencarbery.com/2017/09/grandchild-templates-made-easy/
Description: Demonstrate how to use pagetemplater.php and the included filters
Version: 0.1
Author: Damien Carbery
Author URI: http://www.damiencarbery.com/
*/ 

// Set the directory to search for template files - files will be in 'templates' subdirectory.
add_filter( 'pagetemplater_templates_dir', 'pt_set_templates_dir' );
function pt_set_templates_dir( $templates_dir ) {
    return 'templates';
}


// Remove a template from the list of available templates.
add_filter( 'pagetemplater_templates_found', 'pt_set_templates_list' );
function pt_set_templates_list( $templates ) {
	unset( $templates[ 'templates/does-not-exist.php' ] );
	return $templates;
}


include_once( plugin_dir_path( __FILE__ ) . 'pagetemplater.php' );