I have 2 Oz trail tandems, good for -5c, just lay them out open and put one inside the other.
I used to use my full down Dooner but if it gets wet you are in the crap as someone mentioned.
For a cheap easy fix, Kmart has micro fibre blankets cheap as, they dry very quickly and are very warm, I use up to 3 at home in winter.
They also fold away quite small, also very light, I don`t think you can get better value for money than these.