Abstract: Systems and methods fabricate precast concrete substructures or cores for creating outdoor living structures, such as fireplaces. A mold assembly may include a base section, an outer form section, and an inner form section. The outer and base sections may include pockets for receiving the forks of a fork lift. The outer section may be placed onto the base section, and the inner form section may be suspended within the outer form section. The base section may include three-dimensional (3D) shapes for creating voids in the core to reduce weight. The outer form section may include channels for forming slots in the core, and the base section may include slots for receiving the channels. The channels and slots may be positioned to align the outer form section to the base section as desired.