Assuming that all the holes and edges in the model are drafted sufficiently, then a rigid mold should work. A fiberglass-reinforced polyester or epoxy layup would be lighter, cheaper, and more dimensionally stable than the silicone block you've drawn. A 3D printed mold would not be as strong, since they tend to come apart at the lamination lines. Use several layers of a good mold release and you should be fine.