I am gonna tell you all the way to change Magento default 503 page view so that you can set your site on maintenance without thinking , your site is looking shit now.
Just follow the below simple steps and its too easy :
1- Magento root has errors folder and inside the same you can see folder default.
2- When you open processor.php you see const DEFAULT_SKIN = 'default'.
3- Now change this const value from default to customskin( i am assuming this as new skin name).
4- Copy all files from default folder to newly create folder customskin( under errors folder )
5- Now change inside new skin phtml pages as per your need.
6- You may also edit local.xml as per your need.
Stay Cool !!
Thanks for your valuable comments and suggestions.