This paper reports the optimisation of a PDMS structure for energy harvesting applications. The PDMS structure was optimised using ANSYS simulation. The fabrication processes were also optimised for maximum PDMS ferroelectret energy harvesting performance. The optimised PDMS structure was fabricated using a 3D-printed plastic mould. The ANSYS simulation and experiment results demonstrated the variation in energy harvesting performance of the PDMS ferroelectret depending upon the different void geometry. The optimised operating frequency is determined by the geometry of the voids inside ferroelectrets and the Young's modulus of the PDMS. The peak voltage and energy generated per strike under a compressive load was obtained experimentally as a function of force applied and frequency. The experimental maximum peak output power of an optimised PDMS structure with an area of 2×2 cm2 was 0.46 μW across an optimum load resistance of 21 MΩ under a square wave force of 800 N and 17 Hz.