Human subject studies with mobile users are widely used to understand, and model, human aspects such as behaviours and preferences, in the lab and in the wild. These studies usually employ mixed methods, collecting data by active participation and passive sensing using interactive, mobile, wearable, and ubiquitous devices. Researchers rely on a software platform to design and execute their studies, but existing solutions require a steep learning curve, allow little control, and offer limited guarantees. Our research lab built the mQoL Lab platform using open source technologies, and evolved it to a durable and reliable software ecosystem in over ten mobile subject studies along eight years across three countries. In this paper, we share the acquired experience via tangible artifacts such as requirements, architecture, design, step-by-step support, configuration scripts, and recommendations for researchers to construct a software platform supporting mobile subject studies. The paper is especially relevant for researchers embracing short-term to longitudinal, observational or intervention-based studies, leveraging mixed methods, including multiple devices, and tens to hundreds of simultaneous participants.