Thanks @TheArchitect for the suggestion. My race boots are skin tight without buckling; no room for socks. I managed to squeeze the Sidas boot heaters in by carving out a spot for them in the custom foot bed. Bonus problem, due to the bottoms of my feet being numb, I'm afraid to set the heaters to "high" in case I burn my feet; I've been warned against doing that. They were ok the year before last, but my feet got worse. P.S. It gets cold in Sudbury ON.