I have a Bibler Winter Bivy which weighs about 9 oz. It is made of Epic, if I remember right. It would not be waterproof in a heavy storm and is nothing I would want to be in without a tent or tarp. I use it more for a cover for my down bag than anything else. BD bought Bibler and discontinued it a few years ago. There are some similar designs by other manufacturers.

I have been in an SD Flashlight, in heavy rain. It is a nice small tent but cramped for two unless you are close friends. But, no way I would want to be in a bivy, regardless of design in bad weather unless I also had a tarp. Hammock fans use tarps of various designs. www.hammockforum.com is the place to ask about those.

For the money, I'd rather have a Lightheart or similar tent than a bivy.