I actually have the exact bivy shelter that Frank referenced and it works good. small, compact and lightweight. I used is couple of times hiking the OHT in AR. The only problem I saw is if it were raining and you needed to get in and out...you are basically opening up the entire contents of your bivy to the elements.

I'm a big guy..so to get in this thing you basically set it up..step in at the front, sit down and then scoot yourself down. Not a quick entry or exit, but it serves the purpose. A nice tarp hung up over the head area would cure this problem, but would also add to the weight of the system.