I'm not sure this is answerable without knowing your use case. I have a 30L PVC dry bag backpack I use for caving that's rated for 30 minute submersion at 3 meters but that's going to be super heavy to stick inside your actual backpack. I have this pack which is super light (about a pound) and has a reasonably generous "waterproof" pocket to hold stuff that's either known to be wet or needs to be dry and separate it from the rest of your stuff.