As for the RBI cages, they are great and quite nice to look at as well. Supposedly they hold humidity perfectly, tho my own brief experience was contrary to that. Of course, I did a lot of things wrong in how I set it up the first time, so I'll let you know how things go the second time around once I fix my mistakes.

The bottom of which? The RBI cage or the tub? For the tub, I have a 10gallon sized ZooMed UTH taped underneath each end with foil tape and that does the trick for heat. I need two here because the winters are rather harsh and unpredictable, and even summer can have its colder days.
You can also use Flexwatt under free-standing tubs, and many people prefer it, but whatever you use you must have it on a thermostat of course.