I made one from the clothing type 1443R and have spend maybe 10 nights out in it, the last time the night before last in SE Oklahoma. It weighs about 7 ounces. I am very happy with it. I don't think it would keep me as dry as a real bivy would in the rain, but it did keep me dry on a trip in the Sandia Mountains in New Mexico where the only other person in group not under a tarp woke up soaked from condensation. It got colder than expected that night and I pulled the flap over my head and didn't make any effort to direct my breath outside, still there were no moisture issues. I also used it as a shawl in an unplanned rain/hail storm on Charleston Peak near Las Vegas, it was much more comfortable and easier to regulate temperature with than a plastic groundsheet has been in similar situations. It's also held up well with even though I've never used any kind of ground sheet under it.
The design is very simple: 2.5 or so feet wide by 7 feet long, with an extra-wide floppy section that I can pull over my head. The idea was to prop the floppy section up with something but I've never done that outside of backyard testing, I just use it as a blanket.
I did a test with a drinking glass and a rubber band to see if the material would leak and it didn't after two days. If I make a ball of the material and fill it with water it feels moist, though, even though none will drip out. I think that that's just the way a breathable material is: surface tension keeps water out unless you touch it, like old canvas tents, and in a bivy you'll be touching it so I suspect water would eventually get in in a decent storm. I went out in the yard once during a light rain and read for a while and was OK but I wouldn't want to spend much time like that, especially if it's more than a few degrees warmer or cooler than ideal conditions. For me it's primarily protection from condensation and adds some warmth if necessary. It also doesn't weight much more than an over-sized ground cloth and I can't slide off of it.
A buddy of mine just bought an oversized painting suit of what seems like the same material at Home Depot for about $10 and plans to rip the seams in the legs and glue them to together to make a mummy-shaped version. I'm not sure how that will work but it's a cheap, quick way to get ahold of some material.
Good luck and have fun!