In this work, we describe the design of a customized user interface (UI) for a mixed reality application in a heritage site. The visuals, widgets and spatial interaction techniques have been customized to improve the user experience without diverting the user’s attention by minimizing the effect of the limited field of view (FOV) of the see-through head-mounted display (HMD) used, Microsoft HoloLens v1. The approach consists in using diegetic elements that are coherent with the narrative of the heritage site, and widgets and augmented layers always entirely included in the FOV of the see-through display.