Yes. Your fax cover sheet should be your cover letter; write it as you would a snail-mail cover letter, but make it more concise, if your regular cover letter is long. Make it one page, and I suggest bulleting the key points of your experience that match the job requirements. Format it like a regular fax sheet, using Word's professional or elegant templates.

Just because you are faxing it and not mailing it doesn't mean you don't need a cover letter....