also you can really make a difference by
taking a thin sheet of plastic, cut out a footbed
shape, 2 per foot (a paper protector or overhead transparency)

slap some aluminum foil between them, tape up the sides in a few spots
make an aluminum foil sandwich and stick this on top of your
footbed. it will reflect your heat back to the foot and also help
act as a barrier to the cold coming up from the bottom

again, if it ends up making the shoe too tight all bets are off