The cuben fiber I'm working with is CT0.6K.08 and weighs 15g/sq. meter (0.5 oz). I initially bought it with the purpose of making stuff sacks, and had the idea for rain pants along the way. I dissected a pair of old EMS goretex rain pants that had long seen their glory days. I then laid the parts out on thick plastic and made a mock pair to try on. They fit well, so I was able to use the mock-up parts as a pattern.
I'm using cuben tape for all the seams, and some leftover elastic cord from my tent poles for the waist. The finished rain paints weigh in at a whopping 35.5 grams (1.25 oz)! I think I've achieved absolute minimum wieght on these. Here are some pictures:
Here's a close-up of the taped seams:
and they pack up small too...