This summer, we started to use an unused nylon tent fly from an old 6 man tent as an expedient shelter. The fly is extremely lightweight and is treated with both waterproofing and a fire resistant spray which can be had at many hardware stores.

After 15 or so multi-day lightweight camping and hiking trips, it has held up well with no leaks. The fly folds up compactly and was easy to deploy when we had rain storms or for our nightly shelter. The only downside with not carrying a real tent is the mosquitoes, but that is why we carry bug dope.