Max, if you want a really simple, easy, and very cheap solution read on: Buy one of the heavier duty Frogg Toggs ponchos that has the snaps on the sides. Get some 2 or 3mil plastic--cut to size.
Plastic goes on bottom, completely unsnapped Frogg Toggs goes on top, and stuff some under your feet to keep it there.
If you really want extra protection, create a "bathtub" out of the plastic by folding in the corners, then taping them. This will raise the sides. Easy to do.
To secure the Frogg Toggs poncho onto the top of your body, you can do several things, such as loosely tie some cord around the top of it, tuck some of the very top into your bag or shirt near the collar area, etc
The Frogg Toggs can do double duty as your day raingear too. Its QUITE breathable, definitely more so than even eVent. Also, it's fairly light weight. While i don't like supporting Walmart personally (& try to as little as possible), you can find the heavier duty Frogg Toggs ponchos there for like 10 dollars.
I previously made a bivy via sewing, and using some expensive WPB fabric, zipper, etc, and i really wish i knew then, what i know now, because if i start using tarps i will just do the above since i currently have 2 Frogg Toggs ponchos (one really light weight emergency one, and the heavier duty one with snaps).