Take a piece of tin to put on the snow then put the fire on it, we had a fire on 600mm deep snow all night, and it only melted to the ground in the morning, I also know of guys who used to put the fire tub on star pickets in the snow. Definitely get everybody to bring a big bag of fire wood (will keep the nudie runners warm!!!). I also have put a second thermo rest mattress under my swag.