Making Casings for Prototype Electronic Devices?

kiddailey asks: "I'm working on a project for self-enrichment that involves creating a working prototype of an electronic device. I've made a model of the basic shape of the device out of Sculpey ^(tm) and want to use it as a mold to make the actual plastic housing that I can then insert the electronics into. Has anyone done something like this before? Is there any way to achieve this without a significant R&D buget?"