The increase of renewable energy generated in certain countries has outpaced the expansion of their power grid, causing grid congestion. Currently, grid operators use flexibility measures to counter this challenge. However, these measures struggle to cope with the growth in renewables. There are numerous proposals to improve flexibility measures using distributed energy resources such as electric vehicles (EVs). However, there is a need for a platform whereby EVs can be leveraged directly by grid operators. In answer to the decentralized quality of EVs and the requirements defined by our automotive industry partner, we propose a platform based on a distributed ledger technology (DLT). To achieve this goal, we first designed a concept for a decentralized flexibility market for the stakeholders of the ecosystem. The concept serves as the blueprint for the implementation of the platform. With the design and its implementation and simulation, we validated the use case and technical feasibility of the chosen DLT. We conclude that our prototype has the potential to allow grid operators to leverage idle EVs in aggregation to mitigate congestion.