Shoe covers are the best simple thing. I rode all the last two winters by simply making shoe covers out of plastic shopping bags and duct tape. Put a plastic bag over the toe of the shoe, start wrapping with duct tape, finish by making a loop of duct tape around the heel to hold it on. Cut off the excess bag and cut a hole for the cleats if needed.

Honestly, down to about 30 or so I'm still wearing the thin poly ankle socks that I wear all summer and I've never noticed the cold. This morning it was down to 22*F so I decided to throw on some medium weight full length cotton socks.

I'm considering a DIY electric toe warmer setup for temps below about -10*F where I start getting uncomfortable.