In an emergency, use whatever material is available to form your raft. Man-made materials, like a piece of metal, would work. But, again, remember that the hot metal will melt the snow, so you'll need to stack some logs beneath it. If you don't have a piece of metal handy, the raft can be made of wood--dead, rotten, or green. This last choice is the best of the foraged materials, because live wood has enough moisture in it to keep it from burning for a few hours, but not so much moisture that it puts out your burning sticks or bed of coals. You can use whole chunks of wood, or split the wood in half to create a very flat raft. Obviously, the whole pieces will last longer than split wood. Replace the raft, or pieces of it, as needed.