If you have ever looked at bivy sacks in catalogs or online, you know they can be claustrophobic and expensive. I can't help you much with the first, except to say that you'll get used to it. The expensive part, though, I have a solution for.

I didn't want to spend $200 for a nice bivy, so I bought an "emergency bivy" for $20. It was basically a large plastic bag. I tested it on a rainy night, with a small umbrella over my head. I tried not to breath in the bag, but I still thought I'd be soaked by the condensation, like all the books warn. In the morning I was surprizingly dry.

Later, when I lost my bivy, it occurred to me that if it was basically a large plastic bag, why pay $20 or $200 to replace it? I got out two extra large garbage bags and duct-taped them together. After cutting open one end, I had a three-foot by seven-foot bivy sack. It weighed just four ounces.

Now, if you have looked at bivies before, you know that none are that light. Even my "emergency bivy" weighed twice as much. It was a bit tougher, but then I use my four-ounce bivies as disposables. They are good for a week of nights if you're careful. At less than a dollar each, it doesn't hurt to throw them away at the end of a trip.

Like most bivy sacks, it will leave you a little damp in the morning. It is best used in dry enviroments, although I used mine in Michigan without any real problems. In any case, you'll dry out in a few minutes once you start hiking, and you'll get in the habit of taking a break to lay your sleeping bag in the sun to dry any dampness.

There's our lesson on making ultralight bivy sacks. Four ounces, and they fit in your pocket. This isn't my only disposable lightweight backpacking gear by the way, but that's another story.