Frame and handle from Purple Heart. Solid brass ferrule turned from solid bar. Stainless steel tension rod, adjustment levers and hex nuts. Brass blade holders. Threaded steel handle bolt, epoxied into handle. Aluminium cam lever and brass curved washer. As stated, the frame needs shaping.
What a splendid school project - a mix of skills (wood and metal), not too expensive on materials, and a useful and potentially beautiful end product. I think you're right about shaping - even a few chamfers would 'add lightness' to the appearance. If my 12" bowsaw is anything to go by, it will be a light and nimble tool to use as well.
If it were me (and it might well be at some point!) I think I'd use a screwed rod and wing-nut for tensioning, just because it would then be easier to adjust blade tension in service.
I seem to recall Pete Maddex posting about a (rather elegant) coping saw he'd made, and saying that he preferred his to a bought one. Maybe he could be persuaded to post again....
The Spanish windlass method is simple but the demonstration capabilities of the cam sysem with tap and die cutting of threads, accurate drilling as well as the cutting, shaping and polishing of metal gives them greater experience and demonstration of high level skills, for a 15 year old.