Introductory physics courses often require students to develop precise models of phenomena and represent these with diagrams, including free-body diagrams, light-ray diagrams, and maps of field lines. Instructors expect that students will adopt a certain rigor and precision when constructing these diagrams, but we want that rigor and precision to be an aid to sense-making rather than meeting seemingly arbitrary requirements set by the instructor. By giving students the authority to develop their own models and establish requirements for their diagrams, the sense that these are arbitrary requirements diminishes and students are more likely to see modeling as a sense-making activity. The practice of peer assessment can help students take ownership; however, it can be difficult for instructors to manage. Furthermore, it is not without risk: students can be reluctant to critique their peers, they may view this as the job of the instructor, and there is no guarantee that students will employ greater rigor and precision as a result of peer assessment. In this article, we describe one approach for peer assessment that can establish norms for diagrams in a way that is student driven, where students retain agency and authority in assessing and improving their work. We show that such an approach does indeed improve students' diagrams and abilities to assess their own work, without sacrificing students' authority and agency. (Contains 5 figures.)