Robots can be powerful tools in post-stroke gait rehabilitation. However, state-of-the-art robots are often expensive machines containing rigid links with high inertia. Their expensiveness could limit their availability, and their high inertia reduces transparency, which could hinder rehabilitation progress. These factors raise the need for a minimalistic transparent robot that can effectively fill this gap. This research aims to design and validate such a device, a minimally actuated tendon-based exercise environment. The device is synthesized using an optimization algorithm that considers possible system configurations and optimizes both these configurations and their respective design parameters. Validation is done based on a reconstructed simulation of gait using motion capture on the optimal design to check whether it could be used for real-life rehabilitation. It was found that the most simplistic solution is not yet adequate for rehabilitation; thus, a slightly more complex design is required. While not providing the final solution, this research provides an important stepping stone towards designing a minimally actuated, simplistic, and transparent rehabilitation device.