Joe at Zpacks just made me a bivy in four days flat. I'm super impressed, especially considering that I had a number of mods that I wanted.
If you're interested in a bivy to function as an inner net, I'd definitely recommend asking either Borah or Zpacks, or making your own. They seem pretty simple to make.
For the bottom, the choice seems to be between silnylon or cuben. Depends on how thick your wallet is. Cuben bivvies seem to cost about $170 and weight 4-5 oz, whereas Borah's silnylon/mesh bug bivy is priced at $65 and weighs around 6 oz.
My personal choice was to get a quantum top with a mesh head, because I'll be using it with my Golite Poncho tarp but I want to stick my feet out the end of the tarp if its not raining. This means I can bring only a single trekking pole and still be comfortable, plus I won't be cramped under the tiny poncho tarp as I'm very tall. It also solves problems when I roll around and end up in the rain.
However, there's the condensation issue which we'll have to see about.