Yeah, I have made some mistakes buying various Nomex fabrics before. I prefer a heavier weight plain weave muslin, 10oz/sq yd or so. Stay away from twills or satin weaves. If you get something lighter, make sure it is calendered (rip stop pattern). Also they sell tons of blends now. Don't get fooled into a lower price. You want something where the Nomex and/or Kevlar content is 90% minimum--higher the better.Nomex, and Ebay. Some sellers have light weight, open weave, so be careful what you buy.
I've also used the airbag material. You're right, it's heavy and stiff and will eventually crack, but it's usually free so you can't beat the price.I use airbag material, it is coated so it s non porous and does not fray.
Downside is it is heavy and stiff.