To practice mold logic, I decided to make a pen holder cup on Rhino.
I used the polygon tool to make a 12-sided star and filleted the corners to round them out.
Then I lofted the curves to create a surface I liked, and attached the object to a square base 2mm thick with boolean union. This finished object is my mold positive.
To make the negative, I simply had to use the dimensions of the stock we'll be using on the CNC, which is a foam material, and create a model in Rhino. Once that's done, I took that model and used boolean difference while it was over my mold positive. The result is the mold negative.
I think this object works as a one part mold because it doesn't have any glaring undercuts that could cause problems when slipcasting. It also doesn't have any right angles that could make it hard to de-mold.