I've been admiring your work on your many posts and wonderful pictures.
This bivy project was of interest to me, as I had a last minute invite for a weekender (high winds, rain/snow, 40's-20's in Mass), but no bivy to match my poncho/tarp. Took me about two hours to design and sew one. I purchased some Pertex Quantum from Scottland (last year or so?), and used the silnylon from an unmade Ray-Way tarp. It came out better than I hoped. Even though the trip was cancelled, I went out for an overnighter (30's, 50+ MPH gusts, poured hard rain and snow) and am pleased to annouce I stayed snug-as-a-bug-in-a-rug for about 8 ounces. Thanx for the inspiration!
I sized the hood area to hold all my gear, so I could sleep with my feet on my empty pack. The hood opening runs across at about nose/mouth level, and closes down with an Aircore cinch line held with a tiny spring-loaded-thingie off to one side (so it wouldn't tickle my nose). On each corner is a small grossgrain loop for stake-down points. To make the corners, I just sewed everything else first, then tucked the corners inside while playing with the angles and stake loops, pinned the whole works together and BINGO!
Also, I did try including a loop over the face area of the hood, but placed it too far back. In the field, I found a pebble on the inside with a loop of Aircore on the outside tied up to my poncho/tarp guy worked great. I'm thinking that might be easier and lighter than a balloon.
Also, also, a new friend makes sails for racing yatchs. He saw me making my bivy, and offered me "scraps" of cuben fibre for my next set of projects. I have no way of knowing how large or thick a "scrap" would be, or how many he's talking about, but if anyone is interested then please email me at firstname.lastname@example.org and I'd be happy to share for free.
Thanx again Bill.