my relationship with my seam ripper is definitely love/hate. But she fixes all my problems for me, not without the typical amount of "I told you sews" and angry hostility, but she always gets it done.
If you choose the simple stuff sack plan, which is basically what even more complex bivys are at the core. *You might try having the water proof side larger on the sides and bottom, this should give a bathtub once it is all sewn together (you will need to create a gather at the foot so the seams can meet, this will create the bath tubbing) , to keep the seam and the breathable fabric up off the ground a bit, this only needs to be a couple inches, i have seen some where the whole side is waterproof and I think that they have serious sauna potential*. (worlds longest sentence)