So, that ambient is pretty darn low and I would not be comfortable with it that low for my snakes. Sadly, flex watt does not bump up ambients. Also, rheostats only work well (reliably) if your external temperature (of the room) stays really constant-- like a degree or two of constant.

Hate to sound like a broken record, but the only fix for low ambients that I have found, and that doesn't zap the humidity, is an RHP. You can affix it to a wood top that you make for your cage and then insulate the sides and back of the cage. I think that this would work.