For specific block use unique template file located in custom module's templates folder

Within my custom module 'footermenu' I would like to specify a template file for a specific menu block. I also want to locate the template file within the /templates folder of my 'footermenu' module.

The specific block #id is ‘mainmenu’.

As such I’ve been able to use the function below to change the template file to ‘menu__grid’.

function footermenu_preprocess_block__mainmenu(&$variables) {
  $variables['content']['#theme'] = 'menu__grid';
}

After which (and after clearing cache) it uses 'menu—grid.html.twig' which is all very good.

However, it only searches within the theme’s /templates directory.

How can I make it search within my custom modules templates folder please?

I've tried using the below but it doesn't work in this case.

/**
 * Implements hook_theme_registry_alter().
 */
function footermenu_theme_registry_alter(&$theme_registry) {
  $theme_registry['menu__grid']['path'] = drupal_get_path('module', 'footermenu') . '/templates';
}