A SMES (Superconducting Magnetic Energy Storage) is an attractive power supply for EMRL (ElectroMagnetic Rail Launchers), which require pulse currents to launch projectiles at very high speeds. Moreover, the accelerating force in an EMRL may be enhanced by adding a background field, a principle known as launcher augmentation. The S3EL concept (Superconducting Self-Supplied Electromagnetic Launcher) consists in using the SMES both as a power supply and for background field generation. The challenge is to reach the operating current and peak magnetic field of such launchers, which can be up to hundreds of kA and up to 10 T. The use of High Temperature Superconductor is mandatory to maintain superconducting operation during a launch, though operation at 4.2 K is foreseen to benefit from the REBCO high critical current densities. The aim of this work is to derive a practical design for a 1m-long small-scale S3EL proof of concept using REBCO Coated Conductor, with a reduced output velocity (100 m/s). After preliminary sizing studies, the spatial constraints for the SMES – Launcher integration is presented. A parametric joint analytical and FEM study is conducted to refine the design and maximize the operation margins while maintaining the launch performances. Finally cable design and discharge method suitable to reach the required operating current are presented.