This was inspired by Jamie Short and his bivy design, which can be seen here: http://lytw8.com/My_Gear.html#LytW8_Bivy
I enlarged some of the dimensions a tad, since I am 6', large framed, and have size 13 feet. I also used a longer mesh window, mostly because I intend this to breathe as much as possible. I used a 56" zipper from Zpacks in an "L" shape for the opening. The top and bottom are both 1.3 Sil, it was super cheap at an online store. It weighs in at 9.4 oz and feels pretty durable. All I need to do is sew the seams to box in the corners and its done, but I could not resist snapping some photos after cranking it out in 1 sitting.
Here you can see how wide it opens, and it with my GG Nightlight torso pad and 20 degree bag inside. Its roomy enough for sleep without feeling too claustrophobic, but if it was any narrower at the shoulders it would probably impede my normal thrashing and rotisserie sleeping movements during the night.