Lawnboi
WKR
I guess when your hunting youv got the garbage bag. That's a pretty good idea for the times iv got a garbage bag in my pack
I never really got the waterproofing thing, but its probably because I make my garbage bag pull double duty. I figure I bring it anyways so it might as well serve some other purpose.
I always line my pack with a garbage bag too.
You could also look at the zpacks stuff sacks. Their roll top bags are water proof and very light. (most are under an oz.) http://zpacks.com/accessories/dry_bags.shtml
secret i guess, lol