Although we are not industrial designers, most IoT devices have simple enclosures that we can model and make for easy prototyping. We use onShape and 3D printing to accomplish this.
We will product Parametric models of the enclosure using OnShape. This tool can export in any number of formats that may be needed by the manufacturer. The part(s) will be designed with manufacturing in mind (DFM) to ensure that if the part is going to be injected molded, for example, we haven't defined anything that would have to be redesigned.