I think Tyvek would be a bad choice. All it is is spun HDPE which does not like to stick to stuff. You usually have to corona treat it to stick to an epoxy. It melts at a low enough temperature that it is great for heat sealing.

If you plan on dropping your pack a lot, or throwing it off a cliff, any composite would be a bad choice. If carbon fiber won't work in your application because of stress fracture, then I wouldn't be surprised that any other composite won't work for the same reason. However, if it is just the cost, try replacing it with fiber glass. More detail on what you are trying to do would also be helpful.