Dear (Name of the person who interviewed you), Thank you for considering me for the job opening in your organization. I really believe in your mission and can see the good you do in our community. I would like to volunteer my skills of (list two or three things you can do for them) at these times (list when you would be available to volunteer). I look forward to hearing from you soon. (Your name and contact information here.)
Don't assume they have your contact info. They may destroy applications after the hiring process has ended.