We use these and they are great. I put them on top of the foot and it really helps if you also have toe covers on your shoes. They have a sticky side, so if your shoe is loose enough getting them on shouldn't be a problem.

Note that they don't work once they get wet, so they are less helpful for riding in the rain.

I've also used the back heat patches (you can find them in the grocery store or drugstore - the large ones that cover your back or shoulder or whatever) on long very cold brevets. They can be a huge help in the wee hours when things get VERY cold.