Think Nimblewood only bigger. A metal box is used to confine the fire on three sides thereby using less wood than an open fire, concentrating the heat nicely for a small fire and no scorching the forest floor. Placing a couple of logs under it and the fire box can be used directly on snow pack.
Here pictured with a MSR 2l Alpine pot. The fire box is about 9" wide * 9" high * 12" deep. The box completely disassembles to lie flat for packing - about an inch thick. As I said early this one weighs about 3.3 lbs. The next one will be made from lighter gauge material and a little smaller - all in the never ending effort to shed a few more ounces. Lightweight is all relative to each individuals kit.
Having a fire in winter is almost mandatory - it's the Scout in me.