Woodworkers have to navigate multiple considerations when planning a project, including available resources, skill-level, and intended effort. Do it yourself (DIY) woodworkers face these challenges most acutely because of tight material constraints and a desire for custom designs tailored to specific spaces. To address these needs, we present XR-penter, an extended reality (XR) application that supports in situ, material-aware woodworking for casual makers. Our system enables users to design virtual scrap wood assemblies directly in their workspace, encouraging sustainable practices through the use of discarded materials. Users register physical material as virtual twins, manipulate these twins into an assembly in XR, and preview cuts needed for fabrication. We conducted a case study and feedback sessions to demonstrate how XR-penter supports improvisational workflows in practice, the type of woodworker who would benefit most from our system, and insights on integrating similar spatial and material considerations into future work.