That part (cutting a spheroid to make two interlocking halves) would be the easy part, in a CAD program. Sculpting the dog on top would be hard. I'd say use CAD and 3D printing to make the spheroid, use clay to make the sculpture, and use your molding technique to integrate the two. You don't have to restrict yourself to one-piece molds, though.

Andrew Werby
Juxtamorph.com