What's wrong with flimsy? Do you think that would make it too hard to use, or that it wouldn't be durable? I primarily want it to resist ripping and facilitate a zipper. Thick mylar is available too.

I'm not dead set on a bag either because that adds weight. I'm already planning on having vapor barrier clothing, so I may reuse that to save weight. I briefly looked at Warmlite before, but I'm leaning towards using a rainsuit from Walmart or online motorcycle shop.