I carry a 55 gal, 3mil thick trash bag on the webbing pocket of my backpack.
If the weather looks bad...everything that I'm worried about goes inside the pack, and/or inside that bag. It's then turned upside down with the opening to the bag twisted shut and underneath it all. That all sits under my hammock, under my hex tarp.
If my stuff gets wet after that...it's time to go home.