It's do-able
2 winters ago, I had a little makeshift greenhouse in my yard like Zerah's (mine was 10x16', and about 9-10' tall). I wrapped the entire thing in plastic, mounted an overhead space heater in it, and then had a 150W ceramic heater on each cage (the cages were big, about 3x4x6' tall), but the chams did fine. I had veileds, panthers, Merus and Deremensis out there and was able to see Deremensis breeding in the enclosures in early January 2007 (first time I had ever seen Deremensis breed). The temps in the main room were anywhere in the 60's, and in each cage with the ceramics, the temps were at least in the mid 90's (right under the heater) and around 65 at the cold edges. Our winters are similar to yours, with cold winters getting into the high 20's F. Normal winters are down around 30 degrees F. Just be ready to react if you have a power outage!