Link will not work for me to view it. However I assume someone shows how to make a sleeping bag. I have never personally seen a home made sleeing bag but MANY people make down quilts. In the hammock world they are always making thier own top and bottom down quilts.
Did a little looking, my bag weighs 28 oz already. It's uninsulated so I'd only be adding to the weight. It has a really heavy liner in the inside that really can't be removed. Might have to think of something else.
This looks like a nice idea.
One thing I discovered is that buying a down quilt (for home bed use) isn't that cheep if you want goose down/feather 90/10. The cheapest one I found was $150. The cheaper ones where duck down/feather and often 50/50.
I have an old synthetic bag that I can remove the synthetic filling from and use for this project. The filling is just lumps now so it's useless... it's probably close to 20 years old.