I had some spare sil and dwr ripstop and was thinking about a bivy that was made for a quilt (i.e. no hood).
Depending on the weather, I plan on using my headnet, wpd jacket hood and/or balaclava. All go inside the collar of the bivy which has a draw string closure to cinch everything snug. No critters getting in this bivy.
Weight: 5.8 oz (in silnylon, imagine how light in spin or cuben?)
Size: 36" shoulder X 30" foot X 82" long. The footbox is a little wider than necessary because originally it had a drawstring closure down there too, but I decided to remove that.
Fabric: 1.3 oz silnylon floor, 1.1 oz dwr breathable nylon ripstop top, both cut same size. All seconds from OWF.
Options: (4) Four 1/2" grosgrain webbing tieouts near top and bottom. Boxed foot area to ease pressure on bag loft. Spectra 2 drawstring collar with micro barrel lock.