LOL! Yeah, I figured out how warm they get when my normally tolerant corns were giving me evil looks and laying outside their hide. This is why the rest of my guys are on belly heat but I'm not sure how you would go about doing that on that viv. The plastic holds heat very well and if your thermostat probe is reading air temp, it will always be lower than what your snake is getting and whatever that emitter is directly over will always be hotter. Do you have the stuff to set up the bottom viv like you're going to? Think about how much heat the stuff in the top tank retains and that will keep it warmer than an empty viv.
As for the foil idea, I'm not sure. There are ways to insulate the viv if it is indeed losing that much heat but until you set it up identical to the top, it will be hard to tell whether or not you need more insulation or if it's just the cage setup.