Making is a design-based, participant-driven endeavor based on a philosophy of "learning by doing." Formal and informal educators at science museums, libraries, and universities see Making as a way to engage youth in authentic experiences involving science and technology. However, there is a lack of ethnic, gender, and socioeconomic diversity among Makers. Drawing on research-based principles for engaging underserved communities, we have designed and implemented Mobile Making, a program that operates in afterschool programs at four nearby middle schools. Teams of highly qualified and ethnically diverse undergraduate science and math majors lead weekly sessions that introduce participants to a variety of authentic Making activities. Program objectives target increases in the participants' interest and self-efficacy related to Making and STEM, as well as their perception of the relevance of STEM/Making in everyday life. Observations, focus groups, and surveys have documented positive impacts on the participants and the undergraduate facilitators.