Has anyone here had any luck requesting stage rights to adapt a play/novel/film into a musical?

I've written opera and musical theatre works before, but always with original source material. I have a film in mind to adapt into a musical, and I've spoken to the film company's Rights & Acquisitions department. They told me get the request in writing, and to fax it to them.

Any advice on how to phrase this request to maximize my chances of getting approved?