Here are some pics of a Tyvek bivy sack that I made based off of an old post by Mateusz (which can be found here: Old thread):
Stuff sack made with Tyvek scraps
Bottom of zipper detail
Top of zipper detail
Final weight including the stuff sack is 7.15 oz. I used Tyvek 1443R (the variety used for clothing, 1.25 oz/sq yard) for the entire bivy. Making the bottom out of the more-waterproof Homewrap would probably have added approximately 1.5 oz. However, I regularly use a waterproof padded ground sheet from Suluk46 so I was not too worried about water getting through the floor. I used a #3 YKK locking zipper from Thru-hiker which added about 0.75 oz to the overall weight.
All seams were taped with Tyvek tape cut in half to 1" wide, with the exception of the zipper, which was sewn in with a single straight stitch through 2 layers of Tyvek into the zipper. The Tyvek tape is really strong and with 1/2" on either side, taped both inside and out, the seam is stronger than the material. This also has the advantage of not requiring any seam sealing.
I experimented with some scrap to see what the strongest option for sewing was, and as expected, the fewer holes there were in the Tyvek, the stronger the stitch. So I used a single straight stitch with the longest stitch that my machine could do. This works pretty well, and while it isn't as strong as the tape, I'm not worried about ripping the zipper out any time soon.
I do not have a hood drawstring right now, but I will probably add one as shown in the instructions and Mateusz's pics. The hood is a bit floppy without it.
Overall I am quite happy with how it turned out. It remains to be seen how breathable it is. Some research showed that people have been quite happy with it and haven't had any issues with condensation. It certainly feels quite warm, much warmer than a layer of Momentum 90 would be. I will report back once I try it out a few nights. The total cost was about $45 ($30 for the Tyvek, and $15 for the smallest roll of Tyvek tape, which I used about 1% of) and approximately 5 hours of labor including research.
(I modified this to have a side zip, and changed the height from 15 to 22 cm to accomodate my NeoAir.)