The main challenges for mobile usability labs, as measurement instruments, lay not so much on being able to record what happens on the user interface, but capturing the interactional relationship between the user and the environment. An ideal mobile usability lab would enable recording, with sufficient accuracy and reliability, the user's deployment of gaze, the hands, the near bodyspace, proximate and distant objects of interest, as well as abrupt environmental events. An inherent complication is that the equipment will affect these events and is affected by them. We argue that a balance between coverage and obtrusiveness must be found on a per case basis. We present a modular solution to mobile usability labs, allowing both belt- and backpack-worn configurations and flexible division of equipment between the user, the moderator, and the environment. These benefits were achieved without sacrificing data quality, operational duration, or light weight. We describe system design rationale and report first experiences from a field experiment. Current work concentrates on simplifying the system to improve cost-efficiency.