Most of my outdoor growing was done when I lived in SF and I never had any problems but then again I was usually harvested by now to beat the rainy season. You might try some foam insulation on the inside walls of your grow cab...you can buy it in 4 x 8 sheets in different thickness...1", 1.5", etc.

These days I get nervous when my temps get below 65 but I am spoiled rotten by the weather here in Socal.