Part of the pleasure of being an industrial designer is getting to determine the exterior form of the object you're working on. But let's say you're designing an electronic gizmo--something like the scanner/signing board the UPS guy carries--and you're too consumed with the interface to deal with the form. Then what do you do?
What you do is go to an Enclosures Specialist company like OKW Enclosures, which has the proverbial shit-ton (European translation: "metric ton") of enclosures waiting for you to design guts for. As an example, check out their variety of ABS cases here. And they do more than handheld/arm-mounted--they've got desk-, wall-, rail-mounted enclosures and more.