The Evidence Explained website has a similar query, and a couple of options for such a citation, one brief, and one quite extensive:
Old type citation:
3. Myrtle (Kuhn) Larmore, Writings on the Coon family, photocopy of handwritten manuscript, 1975. Possession of the author.
New type citation with added information in blue:
4. Myrtle (Kuhn) Larmore, Writings on the Coon Family, photocopy of handwritten unpublished manuscript, 1975, Conrad and Barbara Coon Family Research Files; privately held by Elizabeth Ballard (firstname.lastname@example.org), [STREET ADDRESS AND CITY FOR PRIVATE USE,] Indiana, 2016. Contains information on the descendants of Conrad and Barbara Coon as written by Larmore when she was 84. Larmore was a granddaughter of George Kuhn, and great-granddaughter of Coonrod and Barbara. She wrote that she and other family members listened to stories from “Mum,” Sarah (Johnson) Kuhn. Sarah (Johnson) Kuhn was the wife of George. Sarah (Johnson) Kuhn died in 1917, when Larmore was 25 years old. Location of original unknown. Photocopy given to Margaret Ellen Wilson, great-grandaughter of George and Sarah (Johnson) Kuhn and mother of Elizabeth Ballard, by Margaret's brother James William Wilson, circa 1984.