I've been winter hiking with plastic newspaper sleeve bags over heavy wool socks in trail runners for a while, and have just dealt with the bags creeping down around my heel as the day passes. I just did a three-day "experiment" in below 32 degree weather in PA...
day 1, plastic bag over sock in trail runner -- end of day, mostly dry sock, warm feet and bag bunched under my arch.
day 2, figured the bag wasn't doing much and it was cold enough to keep the snow from melting into my sock -- end of day, wet sock and colder feet.
day 3, plastic bag over sock in trail runner, with the excess plastic bag twisted around in the front and then knotted so that I could tie my laces over the knot -- end of day, bag still in place, feet dry and warm... happy camper.
Is there a better light weight way to keep feet dry and warm all day in the snow?