Wiggys makes one out of a material called Lamilite or something like that. It is substantially warmer than a poncho liner. I have both. The Wiggys is substantially more expensive too, but worth it for a cold weather addition to sleeping gear or just to wrap up in.
My poncho liner sees daily use these days on the couch in the basement for watching TV. The Wiggys is what gets thrown in a pack when gear gets grabbed.
