Specify a template for a post type while storing the template file outside of a theme

wp_set_single_template.php

<?php

/**
 * Allows you to specify a template for a post type while storing the
 * template file outside of a theme.
 *
 * @param string $file      Full path to template file
 * @param string $post_type Post Type to use template
 */
function set_single_template( $file, $post_type ) {
	add_filter( 'single_template', function( $single ) use ( $file, $post_type ) {
		if ( $GLOBALS['post']->post_type === $post_type ) {
			if ( file_exists( $file ) ) {
				return $file;
			}
		}
		return $single;
	} );
}