#1 is circulation, make sure your boots are not restricting. There has to be room for alittle air around your foot. You may just have poor circulation in your feet, some folks do. SOme meds will cause this also..

#2 Insulation between your foot and the cold. Sounds like you got that. The carpet idea will help.

#3 Keep them dry. Hard to do in rubber boots but wool will help. Changing your socks will help, liners will help.

For whats it worth, some bootsjustfreeze some folk's feet and not others? Iam lucky and typically don't havecold feet issues. I did however buy a pair of boots a few years ago an gave them away because my feet froze in them no matter what.

good luck