In order to overcome the high computational costs of 3D fluid-thermal topology optimization we introduce a pseudo 3D model consisting of a conductive base-layer and a coupled fluid-thermal design layer. While developed with heat sink designs in mind, the resulting topology optimization approach can be applied to other fluid-thermal design problems where 2D flow effects dominate. We introduce the coupled fluid-thermal solver based on the lattice Boltzmann method, develop and validate the sensitivity analysis for the model, and illustrate the resulting optimization framework for a heat sink design problem.