Improvements in satellite technologies are paving the way for high-powered CubeSats with increasing capabilities. However, due to their small form factor, CubeSats have a limited capacity to dissipate heat loads generated by high power electronics. Phase change material (PCM) heat sinks can enable next generation capability for CubeSats by absorbing the heat loads from high power electronics and dissipating the waste heat to space during periods of downtime. Although, the main challenge with PCMs is overcoming their inherently low thermal conductivity. This study investigated the use of metal additive manufacturing to enhance PCM heat transfer for CubeSat thermal management applications. Numerical modelling was conducted to investigate the performance of additive thermal conductivity enhancement structures for a theoretical 50 W heat sink design dissipating heat from a heat pipe. The selected additive manufacturing structures for the numerical comparison were the strut-based body centred cubic truss and the sheet-based TPMS gyroid. The numerical investigation found that the gyroid sheet-based additive structure provided the best overall PCM heat transfer and also confirmed that smaller base sizes improved performance. Based on the numerical modelling outcomes, a prototype PCM heat sink using the gyroid internal structure was fabricated with copper Bound Metal Deposition and tested in a vacuum chamber using paraffin octadecane PCM. The prototype PCM heat sink demonstrated effective cooling for a 40 W heat load.