Create a template for a custom page in Drupal 7?
Drupal automatically provides path based overrides; so if the path to your custom page is, say, 'custom/page/path' you can create a template file called 'page--custom--page--path.tpl.php`, which will only be used for that page.

Remember to clear Drupal's caches each time you add a new template file or the theme registry won't pick it up.