Last week we had to update all Drupal websites that we manage because of a high risk security release. We saw too late that some (older) sites didn't have a proper maintenance page. So we released a mini module that provides a quick solution for sites with similar issues:
If everything is alright, your Drupal website doesn't have to be in maintenance mode that often. So maybe it's a waste of budget to put effort in this.
But when you dó need to put it in maintenance mode, you probably don't want an ugly page for your visitors.
So that's why we developed this module: after install it'll provide your Drupal website instantly with a clean maintenance page. It doesn't matter what theme you use, it's clean and it works for Drupal 7 and Drupal 8
Saves a lot of time in manually formatting it in template files :)
Check it out: https://www.drupal.org/project/clean_maintenance