Carry several Ziplock FREEZER bags (they're tougher than storage bags). There's an infinite number of ways to use them. The OP suggested using a Mtn House bag as a bed warmer. I suggest putting it inside a Ziplock for extra safety. A wet bed doesn't sleep well.
Ditto on FREEZER Ziplock bags. They are great for many things as mentioned including a spare canteen, phone/camera protection, or even a trash sack.
