Any kind of rope will work. I usually use 3/8" or 1/2" cotton. It's just there to give the glass the shape. Of course if it is soaked in resin it would become real hard. In extreme SPL situations I have heard of people using aluminium rod. I have never tried it. Too afraid it would rattle between the glass. I am going to try it soon though just to see. Guess I'll have to make sure the glass is real tight against it.
Also, I am going to try to make kick panels out of glass for our trucks. I know you can get them in ABS plastic but my dads rattle. I am thinking I will cut into the metal part of the kick and weld in a recessed 'bucket' to get some more room. I think I can get a 8" midbass, a 5 1/4 midrange, and a tweet in there like that. I have an advantage over a truck as I am in a Blazer. For subs I am going to make a fiberglass box for the sides. Each will hold two or three JL 10's. Also in the sides will be recessed amp racks. I will make covers for them so no one will know its there
Later,
Chad