A commercial boot sealer used in cold or wet climates is Sno-Seal, which is made from beeswax. I don't know what else is in it, but I expect beeswax by itself could be used. Perhaps something else could be mixed with it to soften it a bit. I place my boots in the oven and get them nice and hot and rub the commercial product on and it melts and gets absorbed by the leather. Perhaps something like efmesch's furniture polish recipe could be used?