I'm for the heavy duty contractors trash bags for our duluth style packs.  We close them via the twist & heavy duty rubber band method, but I've also improvised with a shorth length of paracord.  Since you can find string at most of the campsites, we've never run out.  Agreed that you should definitely have at least 2 waterproof layers, plus if you absolutely want some items dry, I'd get a small drybag.  I've had great luck with these Pacific Outdoor bags in various sizes, and they aren't too expensive.
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I have one that I stuff my sleeping bag & all of my clothing into, and the purge valve makes it very easy to compress, then put into a pack.