Someone once said that adventures are nothing more than bad planning. By definition, no one ever intends an emergency bivy. But you can plan for it, and the Thermo-Lite 2.0 bivvy sack by Adventure Medical Kits is seven ounces of prevention that can last a lifetime.
The sack, as I shall call it, doesn't take up much space: it's smaller than a quart bottle in the stuff sack. There's enough room to stow a few more essentials in with it, maybe your entire survival kit if you pack efficiently. The bag itself is a basically a 36"x84" weatherproof space blanket with a few trick features. The side opens halfway with Velcro tabs, and the footbox opens as well. This helps manage the inevitable condensation that builds up in what is basically a vapor barrier liner.
It's definitely easy enough to get into and out of, just be careful you don't reef on it. The sack is really meant to be an emergency item and repeated use will tear it up unless you're abnormally careful. It says it can reflect up to 80% of your body heat, and that sounds about right. If you bring an ultralight pad, even a 3/4 length, you'll vastly improve your comfort and heat retention. The Thermo-Lite 2.0 is a nice, inexpensive piece of kit that can epic-proof your outings.