If you want to get a little more fancy than a USGI poncho, you could go with a hammock setup. Start with a silnylon tarp, add a lightweight silnylon hammock with whoopie slings, and then add an underquilt and a small quilt for the top and you have a very comfortable system that can handle wet weather and temps down below freezing and you'll be up off the ground in comfort. In fact you might even find yourself hoping you get stuck overnight so you can use it.

You did say that you are only at about 10k feet so there should still be plenty of trees and a hammock and underquilt sure beats sleeping on the hard wet ground in a bivy that doesn't breathe.

All of what I just described will squeeze down to less than the size of 2 2liter coke bottles and is less than 4lbs. - so it will fit even in something as small as a kid's book pack. For comparison, a USGI poncho, liner and AMK bivy will run you about $100 and a nice silnylon hammock and tarp setup with quilts will run you about $300 (but you can drop it to $150 by building it yourself)

Some others mentioned handwarmers - that's a good suggestion - I'll add that there are 20 hour handwarmers available and they do last 20 hours.

The cookpot suggestion is a good one too. It changes a miserable survival situation into a completely tolerable primitive campout.