Hello fellow California native! I'm in the SF Bay area, so not quite as cold as you, but during the depth of winter my house can drop to the mid to high 60s.

Every single corn I own is on belly heat alone. What I will do sometimes to give them a 'warmer' area is to take a half-log or piece of cork bark, place it on the warm side right above the UTH, and make a cave with several inches of substrate over it. The ambient temperature inside the cave is warmer than the rest of the house and they do great.