If you're talking about sleeping warm, your best bet is a good bag and a good thick pad underneath which is just as important. If you're talking about hanging out, and don't want to go full wall tent mode, a heated tipi from Kifaru or Seek Outside is the way to go. Kifaru has a classified forum and you can pick up a used tent/stove if you're diligent. Kevin T is the man to talk to about Seek Outside.
