The bivi is an off the shelf bivi, the cheapest one I could find cause my bivi-bag-pack is still a prototype.
I won't give details of it nor do I want to show a picture, cause if my dreams come true I would like to put it on the market. Anyone interested? But if you have a look at my avatar picture you'll see sth red behind my back, which is the bivi-bag-pack in pack mode.
I do not believe in a bivi on it's on unless it's below freezing. Making camp in the rain means putting sleepingbag into the bivi in the pouring rain and getting udressed in the pouring rain, so everything will be wet. Sleeping under a tarp will solve these problems. A poncho tarp on it's own also has it's problems, like horizontaly wind driven rain, snow or even worse: sleet. Sleeping under a tarp without bivi at these conditions isn't much fun either. Bivi and tarp together is the best combination and depending on which you buy also the lightest, most versitile set-up. Much more info can be found on this website, that's where I learned all this.