Here's the habitat in it's present form, nearly ready for use:The heating element is that red patch on the wall. I'm thinking it should be wall mounted for a few reasons: First, so the hamster has a choice whether to be in contact with it or not. It's very warm to the touch (not uncomfortably so) and heat transfers very well through the feet; if I put it on the floor the hamster wouldn't have much choice but to walk on it. Also, although (as you can see) I've laid down a layer of kitty litter to handle waste, I'm worried urine might short it out. I could always seal it up with tape so it's moisture proof I guess. Thoughts on placement of the pad are welcome as I really haven't decided yet.
In the picture I'm testing how long it'll run on a 1500mah 5v battery. Still goin' strong after 2 hours, with only 40% of the energy used up. The battery that I'm actually using will be much smaller but roughly the same capacity, so we're looking at about 5 hours of heat. And the habitat should stay warm for a long while afterwards as this type of plastic is an excellent thermal insulator. I'm worried about keeping it cool enough actually, although the constant passage of air through the chamber should have a moderating effect, and by placing the pad on the wall I empower the hamster to regulate his own temperature by getting closer to or further from it.
In the third habitat, if this one is a success (and that appears certain) I'd like to have a power cable as part of the umbilical, and solar panels to keep the air pump, lights and heating pad running indefinitely.