Thru-Hiker has 0.8 oz/sq yd DWR ripstop nylon, but it sounds like you want something lighter. Silk might just be the way to go.
Rockywood Fabrics homemade gear page:
Scroll down on the left side to "Sleeping" and check out 6."Jeff's Bag" That would be a start, for some ideas.
For info on how much insulation to use check out the info on the Ray-Way Quilts kits, on synthetic insulation. Their quilts look scrumptiously warm and cozy. If I weren't using down quilts, I'd probably make one of theirs.