So, I was looking for a sleeping bag in the style of a Big Agnes sleep system bag, but Big Agnes doesn't have the lightest possible materials and the exact size Iwanted.  This sleeping bag has no insulation in the back, instead opting for a pocket where you can insert your sleeping mat. This saves weight, because you're not carrying that extra insulation that would just get compressed and lose its effectiveness. Also, contrasted with a quilt - you can enclose yourself, and roll around quite a lot without feeling the ice-cold stabs of pain as parts of your body leave the warmth of the bag.
I double stitched all of the seams that I wanted to be down proof.
Also pictured are the stuff sack and storage sack.
Recommendations for people considering similar projects:
1) This was not the easiest project to pull off, and I probably spent close to 50 hours getting it together. - Not a great return on investment time-wise.
2) I'd recommend starting with a quilt as a start project to get used to working with down and baffles.
3) Doing a prototype with cheaper material would help your final product look better- you could probably do it at scale to save costs perhaps but then you couldn't get in the bag to make sure it fits.
4) The membrane material is pretty hard to sew, it's very lightweight and slick.
5) Make sure you have plenty of holes in baffles so you can move the down around - I think this approach is easier than weighing the down for each baffle, etc.
