An inexpensive dry bag inside your current backpack works too. 15$ solves the problem. That said, .59c garbage bags work too. I got a new drybag bp for Christmas, and that works as expected. 40 to 200 depending on brand, etc. I put it in my crate on the back for weather free worry sometimes. I do not like overheating so using it on your back may be a deal breaker.