I made my own underblanket from a cheap square sleeping bag from argos, some thick shock cord, and olive green ripstop parachute nylon. Unzipped the sleeping bag, and cut out a rugby ball shape on the diagonal to maximise the length. The excess 'triangles' left from opposite corners of the sleeping bag i then joined together as a square and stitched into the middle of the rugby ball shape to provide double the insulation under the torso. Covered it all in a nice ripstop parachute nylon so it looks bushcrafty, and sewed channels down the sides to run the shock cord through. Shock cord is then tied (very tightly) to my drip rings (karabiners). Works very well, cost me i think around £12 to make with all new materials, and gives the satisfaction of using something you have made!