+1 on the arctic shield boot insulators. They extended my season significantly this year.
I have multiple sits in 20 degrees or colder in my rubber boots this year, all with the boot insulators. My feet sweat profusely, so rubber boots ALWAYS mean damp feet. And damp feet are cold. I've tried changing socks, it didn't matter - my feet just sweat through regardless.
Carry the insulators in with you, when you arrive at your stand, put them on immediately. I found that with my 800 gram boots I wanted chemical hand warmer in there once it dipped below 20.
I wear size 13 alphaburlys in the 800 gram insulation. They barely fit in the largest arctic shield covers they make, but they do fit.