In all my 15 years as a founder, I never engaged an advisor based on a cover latter. So I suggest that it is waste of time. You can get much better results by doing some indept research about the prospect and their pain points, and thinking thru how you can add value with specific strategies/ideas. Think about all variables and multiple options. Then use your social and professional networks to get to meet the key decision makers. Finally make you case. If you do a good job you will get hired!
Good Luck!