Thank you for all the responses. The option I linked has a minimum order of 400', so unless we're all going camping together that's a bit much.
I currently carry two army ponchos in my pack that I have rigged as a rainfly, but I've never been tested on it because the few times I've done it the weather has been good.
I think I'm just going to pickup some 6mm plastic drop cloth at Home Depot today and test it out next weekend on our 2 day hike. I'll let you know how it goes.