Please, if someone tries to make a core and cavity of this Part.
You almost had it all you had to do from there is create a stitched surface with all of your surfaces from one side then create a solid around. Then cut that solid with your surface. After that take the left over solid and insert it into a new part (I always break the referencing when I do this). Depending on your rig it might struggle a bit but this is a fairly simple mold. Repeat for opposite side.