You do not have to worry much about being a copycat if you do not find the most unique cover letter format in town. Just be sure that the contents of the letter have a clear and definite direction. This will bring about a good flow in the contents of the letter.
The letter format should use an appealing font. Maintain uniformity of font and size all through the body of the letter. At the same time, you should create headlines and titles to seem different by simply increasing the font size slightly. Capital or bold letters should be done with care so that reading continues to be smooth. The art of sales writing is time consuming. You could refer to a few sample letters to see how they have made good use of fonts to give a perfect finish.