You might consider making the quilt kit that Ray Jardine sells on his web site, if you think it will meet your needs. You could probably turn it into a bag if you wanted. Or, get the materials from thru-hiker.com or OWF. Synthetic will be cheaper. To make a decent down bag will cost over $100, and you can buy a reasonably good down bag from Campmor for (I think) $109.