This paper proposes and evaluates a rapid mixed reality prototyping process for novel interaction devices. The process consists of five steps: ideation & visualization, CAD modeling, physical construction, MR augmentation, and comparative evaluation. It is exemplarily tested by developing a physical prototype of a transparent handheld display device and evaluating it against an opaque version (N=16). Results show that rapid MR prototyping can detect certain UX differences without the need to actually realize the final device. The transparent version had significantly higher hedonic quality. However, no obvious advantage of transparency could be identified regarding task completion. It turned out that the tablet-based AR solution had limited ability to mediate the technological concept and led to ergonomic problems in the experimental setup. However, the proposed process is flexible enough to allow the use of high-quality MR augmentation solutions such as immersive head-mounted display AR, which could overcome the identified limitations.