This paper presents a novel design and modeling approach for a heat storage device to be used in space applications. This approach is based on phase change materials (PCM) allowing for improved thermal control in nanosatellites. The development of nanosatellites involves many thermal challenges resulting from harsh space conditions and thousands of operational cycles. The greatest thermal challenge is minimizing temperature amplitudes on electronic components due to the ever-changing extreme temperatures and heat flux variations. This is especially challenging in nanosatellites due to mass and volume minimization constraints, low cost requirements, the use of commercial-off-the-shelf hardware, and a low energy budget. PCM-based heat capacitors have been widely used in space due to their ability to store thermal energy and minimize temperature amplitudes on electronic components. However, the design of such devices is complex due to the non-linear nature of the problem, the numerous independent parameters, and the complex heat flow paths and regimes. In this paper, we propose a minimized heat capacitor, designed with an innovative modeling approach that allows for multi-objective optimization based on multi-disciplinary requirements and design freedom arising from the use of state-of-the-art additive manufacturing. We discuss the conceptual design, the proof of concept and the design process, with numerical results showing high fidelity to experiments. This novel approach optimizes the thermal efficiency of the device while minimizing its volume, weight, and cost, leading to a more efficient and reliable satellite, and thus providing a promising solution to the thermal challenges faced by nanosatellites.