What bivy bags are you guys using and how do they do if it starts raining? My tent has dual vestibules and points of entry. This is nice I store gear and food on one side and use the other side to enter and exit the tent. There have been times that it has been difficult to find a goof flat spot to set up camp so I was thinking about a bivy bag. That being said I'm concerned about not having a floor. Any thoughts ?