Research-Practice Partnership (RPP) is recognized as a valuable tool for generating actionable research and improving educational practices by involving practitioners and researchers in both research and the implementation of research findings. However, creating and maintaining such partnerships is challenging; it requires time and financial resources, and a team with diverse expertise and high commitment. Despite their wide recognition in educational fields, RPPs are under-studied in physics education research and literature in informal physics programs is sparse. In this paper, we present a case study of RPP between a physics education researcher and an informal physics practitioner. We describe the development and maintenance of this partnership and use qualitative and ethnographic methods to investigate practitioner-driven concerns about self-evaluation practices and attendance in the program under study, an informal conference for physics teachers. We found evidence that surveys and ethnographic interviews provided complementary data: surveys allowed the practitioner to learn about their audience's motivations, needs, and perception of the program's value. Ethnographic interviews were particularly useful as a means to elicit actionable ideas for how conference organizers might better support teacher attendance. In addition, participating in the RPP provided the practitioner with skills that they can carry forward into their future assessment work and yielded actionable insights beneficial to both practitioner and audience. Our experience suggests that RPPs in informal physics spaces face many of the same challenges as those conducted in formal education settings, and that best practices documented for formal education RPPs will also be of benefit to them.