Good advice so far, especially from MNS on adding extra fabric for the puff-factor. I would recommend you start with the Thru-Hiker wind pants pattern and modify that to fit. I think the name of the pattern is Liberty Ridge wind pants. I would not recommend sewing through down; it wastes down and can make for a sloppy looking seam. You would be much better off quilting ahead and then stuffing each compartment. Of course, this precludes the diamond quilting but I think that diamond quilting is more for fashion than function. If you were to make outer pants and a liner pair leaving the outside seams unsewn, stitch the two together horizontally for quilting (there is no fly on the Liberty Ridge pattern) and then stuff with down, baste and then do a finished seam it would probably work. There would be a lot of fussing and measuring though. Before I tried the above, I would look hard for a pattern. As an alternative, see whether you can locate an old pair cheap and then deconstruct them for a pattern. Have fun and good luck!
May I walk in beauty.