Start-ups tend to form with a central idea that differentiates them from their competitors in the market. It is crucial for them to efficiently transform the idea into a marketable product. Prototyping helps to iteratively achieve a minimum viable product and plays a crucial role by enabling teams to test their ideas with limited resources early on. However, the prototyping process may have wrong focus leading to a suboptimal allocation of resources. Previously, we proposed role-based prototyping for fuzzy front-end development in small teams. It supports (1) resource allocation, (2) the definition of responsibilities, and (3) structuring the development process with milestones. In recent research this was a promising yet incomplete approach. We extend the previous work by refining the prototyping process by adding a prototyping matrix with two dimensions (purpose and lens), a prototyping cycle (plan, execute, test, reflect, assimilate), and a modified Kanban board (Protoban) for planning, managing, and reflecting cycles. This process, named PETRA was tested with a start-up developing an autonomous trash picking robot. The extended approach supported the team significantly in providing a clear idea of what to do at what time.