The Human-Computer Interaction (HCI) space has seen a rise in Spatial Computing applications in recent years, especially with the increasing popularity of Mixed and Augmented Reality. HCI has long recognized the benefit of User Experience measures. However, existing standardized questionnaires are often limited in scope and not well-suited for the continuous evaluation of UX in immersive environments. Building on the Spatial Computing Evaluation Framework (SCEF), we describe the first construction steps of a quantitative instrument, which allows researchers and practitioners to capture the user experiences of Spatial Computing applications, considering the digital, physical, and social environment.