over the years i have used a painters cloth, a blue tarp, a 99cent space blanket and saddle pads.
i liked to froze with a cheap pad the time i used the saddle pads.
I use a chunk of tyvek as well. I have been looking at getting a borah bivy and ditching the tyvek but never know what bivy to get.
Those of you who use a borah which one do you have?