A product is designed in such a way that it is highly functional but the form is entirely reliant on the components pre-existing form - for example an anglepoise lamp's form is shaped by the springs, metal structure and lampshape. There is no additional ornamentation.
I'd suggest Utilitarian
1. of, pertaining to, or characterized by utility.
2. designed for or concerned with utility or usefulness rather than beauty, etc.
This expresses more of an idea that the form arises from the function than that it arises from the components available. If the over-riding influence of the components is what you particularly want to express then Unornamented may be appropriate
1. unornamented - lacking embellishment or ornamentation eg "a plain hair style"; "unembellished white walls"; "functional architecture featuring stark unornamented concrete"