I interviwed with A.T.K, and I don't think you should be using a template, because the recuiters read thousands of CLs, and you want yours to stand out. Just be sure to address 'why ATK?' and 'why you?' Be sure to display a throrough understanding of the firm and their approach to problem solving, thier culture and brand within the industry. If you can swap ATK for McK or BCG in your cover letter, and all statements still hold true, you probably need to customize it a bit more. You could talk about exciting initiatives they have underway, some of their recent work that you admire, etc. You should probably talk to someone in the firm to get more insights.
Here are some links to get started:
All the best!