What constitutes a fairing?

I thought a fairing would be any complete or partial covering intended to improve aerodynamic efficiency. Common solid fairings are GRP, carbon or correx but ripstop nylon and lycra "bag" fairings work too and I don't think there was ever a restriction on a particular method of construction.
As far as I'm aware there is nothing that defines what it can or can't be made of except that if you want to race it or use it on road it would need to have no sharp edges especially forward. And probably not made of anything potentially harmful. You weren't thinking of asbestos were you?

Jst had a crazy idea, to make a frame out of willow, and stretch some of that scaffolding mesh over it. the mesh is a fine mesh that acts as windproofing,  shouldn't weigh much at all.  Just means I'd be slow, in a different class, as you say.

That's an interesting idea. You could do a comparison with a solid fabric to see how they compare. Maybe it doesn't have to be a nice smooth fabric at our speeds? Weight is always important so give it a go!