I acquired a new mattress bag. The things are very heavy mil, probably greater than my commercial 9'x9' tarp. And thats the 'rub' literally. Trashbags, tube tents, space blankets; these are all field expedient, lightwieght and compact items. We are trading greater performance for availability. Theres a point of becoming 'penny wise and pound foolish' when you might as well acquire a dedicated bivy sack. As to fusing, remember you will need some venting for perspiration. I did the following when I played with 55 gallon bags. The bottom bag came up from my feet and I TUCKED the open edge into my pants like an upside down tshirt. The upper bag, with the all important face opening ( this is a survival, not potential body bag.) went over like a loose anorak pullover. The open bottom provides venting. Again, all of these units are great for augmenting a shelter of natrive or carried material. But at some point a dedicated assembly makes more sense.