We present StackMold, a DIY molding technique to prototype multi-material and multi-colored objects with embedded electronics. The key concept of our approach is a novel multi-stage mold buildup in which casting operations are interleaved with the assembly of the mold to form independent compartments for casting different materials. To build multi-stage molds, we contribute novel algorithms that computationally design and optimize the mold and casting procedure. By default, the multi-stage mold is fabricated in slices using a laser cutter. For regions that require more surface detail, a high-fidelity 3D-printed mold subsection can be incorporated. StackMold is an integrated end-to-end system, supporting all stages of the process: it provides a UI to specify material and detail regions of a 3D object; it generates fabrication files for the molds; and it produces a step-by-step casting instruction manual.