Plastic might not be the best choice for the "Panikhacken", unless they come tried and tested of course.
"Piece of string" doesn't sound very stable, is this a rigid design parameter?
Your doc doesn't say anything about size restriction, or how you want it (draft, finished prototype or anything in between)?
Personally, I don't design subsystems without knowing the complete project, as this may have some influence on methods etc. But I'm willing to design an NDA if that would further the revealing of the project.
PM me if you're interested.