Way back in the stone age, 1973, I bought my first mummy down sleeping bag. I cost me a fortune. Well the baffle ends were not closed so a year later when I washed it you can guess what happened. I emptied it and made several items, one of which was a down quilt, which later was stolen.
At any rate with a quilt you have to stuff each compartment indiviually, else the down will shift all over the place when moving it around to sew. You might be able to sew and fill tubes first, then sew the boxes, but it would be difficult with so little down in each pocket, and if you have a problem it will be impossible to repair.
If you want to save yourself some $ and aggravation, just go to joanns and get a pre packaged sheet of 3/8 bonded polyfill (got mine on sale for $5) make the cover, lay it out, pin it, sew the edges, flip it inside out, close the foot, sew the boxes.
Its a lot easier. Mine is made with 1.4 oz nylon from walmart. Entire thing cost me less than $20 but its not full size.
I sleep under mine all the time and 3/8" thick is about right for me with nylon.
If you can find a couple of flat sheets to use for the outer cover, you dont even have to seam the center.