"If you can measure, saw & nail a 2x4, you can measure, cut material & sew a rectangle."
My junior year college roommate bought a sew -it-yourself sleeping bag pattern and materials, including an extra pound of down, from a Seattle mail order merchant called "REI" and made his own sleeping bag. He figured that having made the bag, he'd know how to open the right seam to replace damaged or lost down over the years.
At the forty year class reunion he told me it was a good thing he bought a whole extra pound of down, because he had gone through half of it already.
Dale B. Dalrymple