I made one by cutting the sleeping bag to the width I wanted. Sew on a loop at each corner for a length of shock cord. You can also get fancy and put a cinch cord at the head and foot end. I would recomend doing that. I used 3/4 inch ribbon to make the channel with shock cord fead through it and a cord lock at each end.
It worked ok but I saved up some money and bought a down hammock gear quilt. For weight and pack size it is worth every penny.