I worked for a dude (Texas) who was not only a lousy supervisor, but sadly, he was the owner of the company, and a miserable excuse for a human being, to boot.
I had this unique habit, when I used to work for others (before becoming self-employed): I would, at my first opportunity, steal several sheets of company letterhead and envelopes. Then, when I left the employ of a company, for whatever reason, if the employer refused to tell the truth about me, I would make sure that s/he or they did, with the purloined company stationery.
Just make sure that when you author a letter about yourself, that you first do it on a blank sheet of paper (and that you’re positive, regarding what you wish to say about yourself) before you transfer the text to the official company letterhead.
Have someone else sign the letter, so no one will suspect you, of signing your own recommendation letter.
Finally, mail the letter to yourself, so you have the envelope in which the letter arrived, to prove that the reference letter wasn’t recently written, and enclose a cover letter with your reference letter that reads something like:
Dear Mr. M,
We are pleased to enclose this letter of reference you requested, and wish you well on your future endeavors. You will always have a position here, should you ever change your mind, and wish to re-join our team.
Sincerely,
Jon Dough, Head Asshole