Try reaching out to local business and industry to see if they have their own mentoring initiatives available for employees to implement as part of their volunteer/service opportunities. For example, in our area GE, Corning and TEK-mountain all provide an avenue for their employees to engage in STEM outreach, particularly to encourage girls in STEM. Another source of mentors for girls is sororities that have a STEM focus.
https://ngcproject.org/role-models-and-mentoring