My bonnet was circa 22-23kgs, think it ended up at 11-12 kgs. That was about 7-8mm thick Kevlar.

Even a single skin of kevlar was quite floppy though, and I bonded some 20x20x3mm thick aluminium angle bar on the underside to give it some strength
Or maybe it was 25x25...either way, you get the idea.

So bonding some of that in the manufacture process might be better or easier than trying to make an inner skin from GRP