This seems like a great bivy, exept it...
This seems like a great bivy, exept it wont keep rain out! what is the most light weight way to fix this?
Goto your local building supplier, Home Depot, Lowes, etc and get some Tyvek house wrap... its relatively strong, lightweight, breathes well, inexpensive and can be sewn... Use this to create a fly and a ground sheet!
Another option is a silnylon tarp. That will give you tons of room under it for not only the bivy, but your pack, boots, and even room to cook your meals while you stay tucked away in your toasty sleeping bag. This makes a killer shelter system that's very versatile. It's like a solo tent with a giant vestibule.