The development of student identities--their interests in computer science, perceptions of the discipline, and sense of belonging in the field--is critical for broadening participation of underrepresented groups in computing. This paper reports on the design of portfolios in which two classes of high school students reflected on the process of making electronic textile projects. We examine how students expressed self-authorship in relation to computer science and how the use of reflective portfolios shaped students' perceptions of computer science. In the discussion we consider how reflective portfolios can serve as ideational resources for computer science identity construction.