I made a bivy very similar to that in 2003. My vestibule is larger though...just large enough that I have cooked under it while still inside the bivy during rain. I have stake-out loops at both corners of the top and a pull-out loop right above my head for more head room. The head end and side opening are fully screened. I'm an active sleeper so did not include stake-out loops on the foot end. That way, my lower body has the freedom to curl up, ie, the lower half of the bivy is allowed to move with my body.
The floor and vestibule are 1.1oz sil-nylon and the top some kind of heavy WPB...I believe Outdoor Wilderness Fabric told me Gore-Tex.
The vestibule can be staked to the ground in nasty weather yet the head area remains lifted for ample breathing room.
I used this bivy tons in all kinds of weather and really liked it. I quit using it due to not having enough foot room when I switched over to NeoAir. I was also wanting something lighter as it weighs 18oz. I'm guessing with the use of cuben for the floor and vestibule as well as a lighter WPB top, I could get it down to the 13-14oz range. But, it's a bit more of a project than I want to tackle now...plus, I have been enjoying the extra space of my modified Hexamid.
Anyhoo, just thought I'd share my experience with ya. Have fun with it!