I am designing a mold box that I want to 3D print and use for blue stuff casting, and I have come up with this design. It's basically an inner box with unconnected walls that slot into the base, that are then enclosed by an outer box that keeps everything together. On the top there's a push block so I can clamp it down and apply pressure to the mold.

How would I go about formalizing this design? I started with Autocad but it has issues parametrizing 3D objects, I want to be able to set variables so I can alter sizes and tolerances without redoing the whole thing. Any tips on materials or how to go about tolerances?