The purpose of the bison is for use in sub zero temps for multiple days on mountains without a tent. Unless I can find a tent for winter use weighing less than a bivy.

I also have an old tennier black sleeping bag from my former years. The rest of the system is somehow lost. I don't plan on bringing it with me on these winter mountain trips.

The backpack fits me well and is rather comfortable. At 5 lbs I wonder if going lighter would be better. I don't like how I am unable to fit the bison sleeping bag in horizontally verses vertically. I could save more space that way.

Thank for the advice on the plastic bags.
I will keep looking for a tent. I wonder if there is a pack with a wider base to accommodate the space of the bison being laid horizontally.