I live just North of Toronto - so I know what cold you're talking about! brrr...

I have 7' x 24" x 24" enclosures. I have 12" wide heat tape covering the ENTIRE floor, half of the enclosure is regulated to be 90 degrees and the other half is regulated to be 81 degrees. By doing that, no matter what the ambient temps are, the floor temps are always correct.

If that still isn't enough, 1 or 2 low wattage ceramic bulbs can raise the ambient temps a bit.