Questoutfitters.com has a bivy pattern. Look under "patterns" and then tents/shelters".
Observations about the above pattern. There is a lot of material in the head area. Its rectangular shape makes it easier to sew, but a rounded head would save material and weight. Rather than have the zippers go up the sides of the head box, have a zipper go straight across the top, and another zipper, or an extension of the same zipper, go maybe two feet down the side. Waterproof zippers would eliminate the need for flaps over them. Also it does not have the option of rolling back or removing the mosquito netting. The body of the bivy looks good. Remember, too, that a bivy has to easily fit over your sleeping bag, and consider the loft of the bag you are most likely to use with it. In other words, the bivy needs to be a "baggy" fit, not a tight fit. I actualy have Momentum90 from thru-hiker and silnylon waiting to be made into a bivy. Momentum is soft, with a very nice feel to it. The red looks great with black silnylon! The shiny side goes on the inside as I was advised at thru-hiker. The material is sold by the yard, not segments of yards. I think I will be able to make the bottom one piece with a curved head box included. I will consider having the top piece taper in at the bottom, but include a circular "head" which will become part of the foot box. Maybe not, however, because I do not want the fotbox too tight as mentioned above. I also want to be able to roll back the netting and close it with a zipper, not velcro. I might not even add netting.