Im going to give it a whirl. Im thinking that I can take my model, then create a box around it, then delete the model from the inside of the box which should then give me my mold. Only thing that i am worried about is overhang inside of the box being that I do not want any infill where the model previously was. Im guessing that I will also have to design some holes in the top/bottom of the mold in order to give me a place to pour the liquid into the mold and also to help with bubbles, etc.

I will report back what I find out, seems like a fun challenge to tackle.