How to create a custom template for entities?
You don't need to add entity string to the template file name. These are theme hook suggestions for entity built with ECK (assuming your entity and bundle names):


section.tpl.php 
section--masthead.tpl.php
section--masthead--full.tpl.php
section--[ID].tpl.php


As you can see there is a naming conflict because section is a drupal builtin template. It would be safer to use different name for your entity.