Last winter I cut off the corners of ziplock bags and put them over my toes betwixt two pair of smart wool socks. I then taped off the openings on top of my sidis. After that I put on some lined shoe covers.
Coldest ride was slightly below 20F on my 2+ hour commute to work. Not too much of a problem.
This year I'm going to find something that doesn't take half the morning to do.