Edge computing requires the complex software interaction of geo-distributed, heterogeneous components. The growing research and industry interest in edge computing software systems has necessitated exploring ways of testing and evaluating edge software at scale without relying on physical infrastructure. Beyond simulation, virtual testbeds that emulate edge infrastructure can provide a cost-efficient yet realistic environment to evaluate edge software. In this experience paper, we share lessons learned from building a total of five edge software testbeds. We describe pitfalls in architecture and development as well as experiences from having students use our testbed tooling in distributed systems prototyping classes. While we remain confident that building custom testbed tooling is the right approach for edge computing researchers and practitioners alike, we hope this paper allows others to avoid common mistakes and benefit from our experience.