I was up in the Arctic for this years solar eclipse we had temps down to possibly -30C at times and for the 3 hrs we spent out at site the temperature went from -16C to -23C back to -16C.
Also stood out on the snow and ice at -10C to -15C for 6 hours one night watching auroral displays.
I bought a pair of Neos insulated overshoes. You wear them over other boots. Feet were toasty warm the whole time. Never felt cold once. To keep the insides clean, I'd recommend you put a separate pair of warm boots or shoes inside them and have another pair for driving to the site and walking around unpacking etc. They are big and not great for walking around, carrying stuff. better to put them on after you are set up.
Goto my web page
http://www.joe-cali.com/eclipses/PAS...015/index.html
There is a link at the bottom to "technical descriptions" which has a link to the shoe supplies. There are other brands of overshoe available. I have big feet- Euro size 48. Neos had the biggest shoes and the only ones that only just fitted over my boots.
Note that you have to physically measure the heel to toe length of the boots you want to wear inside the overboots then look at the sizing chart and select an overboot big enough to fit over your boots. Also, I could not purchase direct, had to use an online personal shopper service to get them & freight to me in Australia. The website has the link to the shopping service but it did almost double the price of the boots. Think I paid about A$210 for them.
Joe